[vtctldserver] Remove parallelization from tests that mutate shared state

[ajm188]

Description

This removes t.Parallel from the test cases that mutate the stubbed out "get version" function, which trips the race detector.

VTAdmin integration tests also indirectly hit this, because testutil.BuildIntegrationTestCluster instantiates a grpcvtctldserver.Server, which can race. This is why I believe we've been seeing vtadmin tests showing up as flaky in launchable.

Related Issue(s)

N/A

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

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[frouioui]

Merged main to make vreplication_across_db_versions go green :)

[vitess-bot]

I was unable to backport this Pull Request to the following branches: release-16.0.

[deepthi]

I was unable to backport this Pull Request to the following branches: release-16.0.

This test failed on #13290 (release-16.0), so I'm applying this fix manually into that PR. There is no longer a need to do a separate manual backport.