release-25.2: 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 #158071 is compliant with backport policy

Confidence: high
Backward compatible: true
Explanation: The pull request is compliant with the backport policy because it contains changes that are limited to a test file, specifically pkg/jobs/registry_test.go. Considering the file naming convention, it clearly falls under the non-production file patterns outlined in the policy for test files (*_test.go). Additionally, the PR body includes a 'Release justification: test only change', which explicitly cites that the changes are related solely to tests, further confirming the exemption from standard backport policies. The PR aims to fix a testing issue, which is evident from the changes and comments that adjust test behavior, thereby making it a non-production code change that doesn't require examination of further compliance elements such as critical bug fixes or feature flags.

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[cockroach-teamcity]

This change is