Avoid flaky topo concurrency test

[timvaillancourt]

Description

This PR avoids flakiness in the TestStatsConnTopoListDir and TestStatsConnTopoGet unit tests of go/vt/topo by ensuring the semaphore we hold to test semaphore-wait stats is acquired before testing a ListDir or Get

I don't see cases of this flakiness on our fork or upstream, but I think flakiness possible in extreme cases

Also, test logic was updated to use require.Equal, require.(No)Error and require.(Not)Zero

Related Issue(s)

Checklist

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

Deployment Notes

[vitess-bot]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.69%. Comparing base (b1f6147) to head (5c561d8).
Report is 17 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17407      +/-   ##
==========================================
+ Coverage   67.66%   67.69%   +0.02%     
==========================================
  Files        1584     1584              
  Lines      254360   254505     +145     
==========================================
+ Hits       172116   172288     +172     
+ Misses      82244    82217      -27     

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