Flakes: Address flakes in TestMoveTables* unit tests

[mattlord]

Description

Since #16583 we've seen the TestMoveTablesUnsharded and TestMoveTablesSharded tests become flaky. Especially the Sharded one.

The only directly relevant change made to these unit tests in that PR is adding the vrdbClient.Reset() call. That call however is not safe as it modifies the members while holding a mutex, most notably the done channel. The problem, however, is that we explicitly and intentionally do not take that mutex in the Wait() function, which is the primary user of that channel.

It's not 100% clear that this was the cause of the flakiness, but given that it was the only directly relevant change, we know it's unsafe, and CI failures that I saw (I was unable to repeat the test failures locally) seemed to indicate a repeated panic->recover cycle in the TabletManager which could be caused by unsafe channel manipulation:

ok  	vitess.io/vitess/go/vt/vttablet/tabletconn	1.035s
W1009 13:10:59.004444   47317 log.go:39] Failed to read in config : Config File "vtconfig" Not Found in "[/opt/actions-runner/_work/vitess-private/vitess-private/go/vt/vttablet/tabletmanager]". This is optional, and can be ignored if you are not using config files. For a detailed explanation, see https://github.com/vitessio/vitess/blob/main/doc/viper/viper.md#config-files.
W1009 13:10:59.005626   47317 log.go:39] Failed to read in config : Config File "vtconfig" Not Found in "[/opt/actions-runner/_work/vitess-private/vitess-private/go/vt/vttablet/tabletmanager]". This is optional, and can be ignored if you are not using config files. For a detailed explanation, see https://github.com/vitessio/vitess/blob/main/doc/viper/viper.md#config-files.
W1009 13:10:59.026628   47317 log.go:39] Failed to read in config : Config File "vtconfig" Not Found in "[/opt/actions-runner/_work/vitess-private/vitess-private/go/vt/vttablet/tabletmanager]". This is optional, and can be ignored if you are not using config files. For a detailed explanation, see https://github.com/vitessio/vitess/blob/main/doc/viper/viper.md#config-files.
W1009 13:10:59.027266   47317 log.go:39] Failed to read in config : Config File "vtconfig" Not Found in "[/opt/actions-runner/_work/vitess-private/vitess-private/go/vt/vttablet/tabletmanager]". This is optional, and can be ignored if you are not using config files. For a detailed explanation, see https://github.com/vitessio/vitess/blob/main/doc/viper/viper.md#config-files.
E1009 13:10:59.057000   47317 server.go:959] buildTrafficSwitcher failed: no streams found in keyspace sourceks for testwf
--- FAIL: TestMoveTablesSharded (0.03s)

...

W1009 13:10:59.058921   47317 log.go:39] Failed to read in config : Config File "vtconfig" Not Found in "[/opt/actions-runner/_work/vitess-private/vitess-private/go/vt/vttablet/tabletmanager]". This is optional, and can be ignored if you are not using config files. For a detailed explanation, see https://github.com/vitessio/vitess/blob/main/doc/viper/viper.md#config-files.
W1009 13:10:59.059540   47317 log.go:39] Failed to read in config : Config File "vtconfig" Not Found in "[/opt/actions-runner/_work/vitess-private/vitess-private/go/vt/vttablet/tabletmanager]". This is optional, and can be ignored if you are not using config files. For a detailed explanation, see https://github.com/vitessio/vitess/blob/main/doc/viper/viper.md#config-files.
W1009 13:10:59.076345   47317 log.go:39] Failed to read in config : Config File "vtconfig" Not Found in "[/opt/actions-runner/_work/vitess-private/vitess-private/go/vt/vttablet/tabletmanager]". This is optional, and can be ignored if you are not using config files. For a detailed explanation, see https://github.com/vitessio/vitess/blob/main/doc/viper/viper.md#config-files.
W1009 13:10:59.077371   47317 log.go:39] Failed to read in config : Config File "vtconfig" Not Found in "[/opt/actions-runner/_work/vitess-private/vitess-private/go/vt/vttablet/tabletmanager]". This is optional, and can be ignored if you are not using config files. For a detailed explanation, see https://github.com/vitessio/vitess/blob/main/doc/viper/viper.md#config-files.

...
E1009 13:10:59.179155   47317 dbclient.go:134] error in stream 1, will retry after 5s: no schema defined
W1009 13:10:59.179357   47317 controller.go:197] context canceled: no schema defined
W1009 13:10:59.180705   47317 log.go:39] Failed to read in config : Config File "vtconfig" Not Found in "[/opt/actions-runner/_work/vitess-private/vitess-private/go/vt/vttablet/tabletmanager]". This is optional, and can be ignored if you are not using config files. For a detailed explanation, see https://github.com/vitessio/vitess/blob/main/doc/viper/viper.md#config-files.
W1009 13:10:59.181630   47317 log.go:39] Failed to read in config : Config File "vtconfig" Not Found in "[/opt/actions-runner/_work/vitess-private/vitess-private/go/vt/vttablet/tabletmanager]". This is optional, and can be ignored if you are not using config files. For a detailed explanation, see https://github.com/vitessio/vitess/blob/main/doc/viper/viper.md#config-files.
W1009 13:10:59.189298   47317 controller.go:187] context canceled: no schema defined
W1009 13:10:59.190224   47317 log.go:39] Failed to read in config : Config File "vtconfig" Not Found in "[/opt/actions-runner/_work/vitess-private/vitess-private/go/vt/vttablet/tabletmanager]". This is optional, and can be ignored if you are not using config files. For a detailed explanation, see https://github.com/vitessio/vitess/blob/main/doc/viper/viper.md#config-files.
W1009 13:10:59.190957   47317 log.go:39] Failed to read in config : Config File "vtconfig" Not Found in "[/opt/actions-runner/_work/vitess-private/vitess-private/go/vt/vttablet/tabletmanager]". This is optional, and can be ignored if you are not using config files. For a detailed explanation, see https://github.com/vitessio/vitess/blob/main/doc/viper/viper.md#config-files.
E1009 13:10:59.200891   47317 dbclient.go:134] error in stream 1, will retry after 5s: no schema defined
W1009 13:10:59.202169   47317 controller.go:187] context canceled: no schema defined
W1009 13:10:59.203078   47317 controller.go:197] context canceled: no schema defined
W1009 13:10:59.204084   47317 controller.go:187] context canceled: no schema defined
W1009 13:10:59.205748   47317 log.go:39] Failed to read in config : Config File "vtconfig" Not Found in "[/opt/actions-runner/_work/vitess-private/vitess-private/go/vt/vttablet/tabletmanager]". This is optional, and can be ignored if you are not using config files. For a detailed explanation, see https://github.com/vitessio/vitess/blob/main/doc/viper/viper.md#config-files.

...

The Reset() function was being called for every shard, so that would also explain why the Sharded test was much more flaky than the Unsharded one.

Given all of this AND the fact that using this Reset() function is wholly unnecessary we remove its usage in this PR. We will then have to see how much this helps the somewhat random flakiness we've seen in the coming weeks.

While here, I also added mutex usage to the fakeTMClient methods that weren't already using it so that we should be concurrent safe if we start doing more tests in parallel in the future.

unit_race test re-runs can be seen here: https://github.com/vitessio/vitess/actions/runs/11317748149/job/31471559199?pr=16942

Related Issue(s)

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.40%. Comparing base (f0062e6) to head (c14175c).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16942      +/-   ##
==========================================
- Coverage   69.41%   69.40%   -0.02%     
==========================================
  Files        1570     1570              
  Lines      203948   203942       -6     
==========================================
- Hits       141576   141542      -34     
- Misses      62372    62400      +28     

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