CI flaky test: Fix flakiness in vreplication_migrate_vdiff2_convert_tz

[rohit-nayak-ps]

Description

Fixes two flaky tests:

1. Address theVDiff stop test which also regularly fails locally. The problem here is that we are trying to stop a vdiff which completes before it can be stopped. vtctldclient is generally much faster and we have virtually no data to diff so there's not really any way to avoid this race. So we consider stopped and completed to be valid/acceptable.

2. Fixes TestVtctldMigrate e2e test.
   After VReplication: Improve workflow cancel/delete #15977 the target also has denied tables and hence we need to query the tablets directly rather than check via vtgate.

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
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Deployment Notes

[vitess-bot]

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[mattlord]

Thank you, @rohit-nayak-ps ! ❤️ I'll re-run the workflow a few times and then, assuming it passes each time, go ahead and merge it.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.58%. Comparing base (8a01d03) to head (3247679).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16180      +/-   ##
==========================================
+ Coverage   68.54%   68.58%   +0.03%     
==========================================
  Files        1544     1544              
  Lines      197737   197750      +13     
==========================================
+ Hits       135532   135620      +88     
+ Misses      62205    62130      -75     

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