release-25.2: roachtest: mark hibernate/testImplicitJoining as flaky

[shghasemi]

Backport 1/1 commits from #160310 on behalf of @shghasemi.

---

Fixes: #160299

Release note: None

---

Release justification:

[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 #161451 is compliant with backport policy

Confidence: high
Backward compatible: true
Explanation: The pull request modifies a file classified under 'development tools' for CockroachDB. Specifically, it adjusts behavior in 'pkg/cmd/roachtest/tests/', which is designed for roach testing infrastructure—a non-production environment. The changes in this area are thereby exempt from needing direct justifications like a critical bug fix or feature flag as outlined in the backport policy, so long as they only impact test execution or configuration.

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

[cockroach-teamcity]

This change is