VReplication: Restore previous minimal e2e test behavior

[mattlord]

Description

In #16920 we changed the behavior of the minimal cluster setup so that it went from using a hardcoded value of 0 for the replica and rdonly tablet counts, to instead using the default globals for that (defaultReplicas, defaultRdonly): https://github.com/vitessio/vitess/pull/16920/files#diff-127f033346a57c5e68aaa42072891ad18fca2bd1463eb16cd16a23f74b136576R942

When working on that PR I noticed that the buffering test started failing every time and the cause was that it now had replicas (and rdonly tablets) and they could not keep up with the load being generated against the primary. So (v)replication could not catch up in time and the vdiff did not finish before the 30s timeout. I addressed that in the PR here: a629cbb

What I didn't notice was that this minimal cluster behavior change also caused the TestPartialMoveTablesBasic test to become flaky (not failing every time, but ~ 50% as I watched the last test run on that PR and the subsequent backport PRs). The failures were caused by the same basic problem, we now had replication in place and the replicas could not catch up with the primary fast enough for the vdiff to complete before the 30s timeout we use in the tests.

In this PR we explicitly ensure that the test behavior for users of the setupMinimalCluster(t) remains the same after the change noted from #16920 (the exception is TestVtctldclientCLI in which we explicitly want to have a replica tablet in each shard to test traffic switching). We should backport this to v21 as #16920 was backported to v21.

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.16%. Comparing base (6c6499c) to head (f0a233b).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17016      +/-   ##
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+ Coverage   67.14%   67.16%   +0.02%     
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  Files        1571     1571              
  Lines      251785   251785              
==========================================
+ Hits       169049   169108      +59     
+ Misses      82736    82677      -59     

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[shlomi-noach]

Nit: "og" naming in this function as compared with "orig" naming in other functions. I prefer the latter FWIW.

[rohit-nayak-ps]

We are using this pattern cut-pasted in many places. This is going to be refactored soon into calling a function that changes the number of replica/rdonly count and returns a deferable reset function. (Or we will remove the globals and make them test-scoped)

So merging as-is for now to get the backport ready well in time for RC2.