release-24.3: bench/rttanalysis: shard TestBenchmarkExpectation to avoid timeouts

[rickystewart]

Backport 1/1 commits from #157847.

/cc @cockroachdb/release

Release justification: Non-production code changes

---

Re-apply 9fecc53b0b3cde307b5379ee5b88fae0fc8f34e2, but add a skip if the test is running under s390x.

Release note: none
Epic: none

[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.

[cockroach-teamcity]

This change is 

[blathers-crl]

✅ PR #157958 is compliant with backport policy

Confidence: high
Backward compatible: true
Explanation: The PR qualifies for an exception due to the inclusion of a release justification statement and changes to non-production files focused on testing. Specifically, the PR body includes a 'Release justification: Non-production code changes' line, indicating that the changes should not affect production code and thereby exempt from the strict backport policy. The files changed are within the pkg/bench/rttanalysis/ directory, which is related to benchmark tests and not part of the production deployment. These files pertain to modifications and improvements in testing, particularly affecting the benchmark testing of the project.

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