release-23.2: storage: fix TestMVCCStatsRandomized's handling of timestamps

[miraradeva]

Backport 1/1 commits from #122976.

/cc @cockroachdb/release

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Recently, #113306 attempted to fix an issue in TestMVCCStatsRandomized where a transactional request was silently failing because its timestamp did not match the transaction's read timestamp. That patch handled many of the types of requests in TestMVCCStatsRandomized but not all (e.g. transactional GC requests).

This patch addresses the issue by refactoring the timestamp manipulation to a single place in the test.

Fixes: #119409

Release note: None

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Release justification: Test fix

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

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[cockroach-teamcity]

This change is