Flaky test fixes

[GuptaManan100]

Description

This PR fixes some of the tests that we've noticed to be flaky in the CI.

• TestTabletExternallyReparented - In the check for the final list of tablets, we were using a context with a 10ms timeout. That is very low, even if all the topo calls are local (test uses memorytopo). In some cases, the final call to get tablets for a cell will expire, leading to incomplete list and a failure like so -

2024-09-11T23:58:45.2613127Z === NAME  TestTabletExternallyReparented/success
2024-09-11T23:58:45.2613346Z     proto_compare.go:118: []: (-want +got)
2024-09-11T23:58:45.2613553Z           []*topodata.Tablet{
2024-09-11T23:58:45.2614267Z         - 	s`alias:{cell:"zone1" uid:100} keyspace:"testkeyspace" shard:"-" type:PRIMARY primary_term_start_time:{seconds:1000}`,
2024-09-11T23:58:45.2614829Z         - 	s`alias:{cell:"zone2" uid:200} keyspace:"testkeyspace" shard:"-" primary_term_start_time:{}`,
2024-09-11T23:58:45.2615263Z           	s`alias:{cell:"zone3" uid:300} keyspace:"testkeyspace" shard:"-" t`...,
2024-09-11T23:58:45.2615396Z           }

The fix is to increase the timeout.

• TestListenerShutdown - After we shutdown the listener, we are immediately checking for connRefuse to have a value of 1. This can however take some time, because it is asynchronous. After we call Shutdown, we Close the underlying net.Listener but it doesn't guarantee that we'll see an error in Accept() immediately. So waiting for connRefuse to get to 1 fixes the problem.

• TestPlannedReparentShardPromoteReplicaFail - After the recent changes wherein we've started querying PrimaryPosition from SetReplicationSource, we have seen some data races, because calls to ERS will spawn a go routine for SetReplicationSource which can keep running after ERS ends. If then we try to change the primary position, we see a data race. This PR fixes this by adding mutex protection when accessing the PrimaryPosition field.

Related Issue(s)

Checklist

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.39%. Comparing base (f0062e6) to head (a75c834).
Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16940      +/-   ##
==========================================
- Coverage   69.41%   69.39%   -0.03%     
==========================================
  Files        1570     1570              
  Lines      203948   203976      +28     
==========================================
- Hits       141576   141547      -29     
- Misses      62372    62429      +57     

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[mattlord]

The PR title and description make it seem like you intended to include multiple fixes. Is that the case?

[mattlord]

Nice! Thank you, @GuptaManan100 ! I just had the one nit that I think is worth doing, but up to you.