Add locking to mount tests that write to the mount table

[VioletHynes]

Description

This was causing a data race in tests due to some new code that runs and reads the mount table.

Will need to backport this to N-2.

TODO only if you're a HashiCorp employee

• Backport Labels: If this fix needs to be backported, use the appropriate backport/ label that matches the desired release branch. Note that in the CE repo, the latest release branch will look like backport/x.x.x, but older release branches will be backport/ent/x.x.x+ent.
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  break any tests. Sometimes ENT only tests rely on public functions in CE
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[mpalmi]

Sorry for the drive-by review, but I would be curious to see which code introduced the new data race. Historically new races like this were introduced due to goroutine lifecycles living outside of seal/unseal. Probably not an issue here, but figured I'd bring it up just in case...

[github-actions]

CI Results:
All Go tests succeeded! ✅

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Build Results:
All builds succeeded! ✅

[stevendpclark]

👍