release-24.3: jobs: fix TestJitterCalculation failed

[blathers-crl]

Backport 1/1 commits from #157951 on behalf of @jeffswenson.

---

This test will sometimes fail due to a randomly picking a value that
rounds to 1/2. The frequency of this is a bit higher than one would
expect because the rounding behavior depends on the width of the
duration, not the width of a float64. The test was adjusted to retry
if the jittered value equals the input.

Release note: none
Fixes: #128381
Fixes: #157113

---

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 #158068 is compliant with backport policy

Confidence: high
Backward compatible: true
Explanation: The pull request is compliant with the backport policy for non-production changes. The PR only modifies a test file (pkg/jobs/registry_test.go), and the PR body explicitly states a 'Release justification: test only change.' According to the backport policy, changes that only affect non-production code, tests, or development tools do not require a critical bug justification or feature flag gating and are exempt from standard backport policy requirements.

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[cockroach-teamcity]

This change is