release-24.3: roachtest: simplify and deflake cleanup in clock jump test

[tbg]

Backport 1/1 commits from #156369 on behalf of @tbg.

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Closes #156339.

Passing test run: https://teamcity.cockroachdb.com/buildConfiguration/Cockroach_Nightlies_RoachtestNightlyGceBazel/20660221?hideTestsFromDependencies=false&hideProblemsFromDependencies=false

Epic: none

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Release justification: Test-only change.

[blathers-crl]

Thanks for opening a backport.

Before merging, please confirm that it falls into one of the following categories (select one):

• Non-production code changes. Includes test-only changes, build system changes, etc.
• Fixes for serious issues. Defined in the policy as correctness, stability, or security issues, data corruption/loss, significant performance regressions, breaking working and widely used functionality, or an inability to detect and debug production issues.
• Other approved changes. These changes must be gated behind a disabled-by-default feature flag unless there is a strong justification not to.

Add a brief release justification to the PR description explaining your selection.

Also, confirm that the change does not break backward compatibility and complies with all aspects of the backport policy.

All backports must be reviewed by the TL and EM for the owning area.

[blathers-crl]

✅ PR #156526 is compliant with backport policy

Confidence: high
Backward compatible: true
Explanation: The PR modifies a file within a directory explicitly designated as non-production code as per the CockroachDB Backport Policy, specifically pkg/cmd/roachtest/. The changes are to a file used solely for testing (tests/clock_jump_crash.go), thus adhering to the exemption for non-production and test-only changes. Additionally, the PR includes a 'Release justification: Test-only change.' which further supports its exemption from the standard backport policy requirements.

_{🦉 Hoot! I am a Blathers, a bot for CockroachDB. My owner is dev-inf.}

[cockroach-teamcity]

This change is