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

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Codecov Report

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Additional details and impacted files

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