release-25.2: restore: deflake TestRestoreCheckpointing

[blathers-crl]

Backport 1/1 commits from #154654 on behalf of @kev-cao.

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The current TestRestoreCheckpointing waits for job progress to be updated by sleeping for a duration greater than the checkpoint interval. While this works the vast majority of the time, if the test cluster is overloaded, it is not sufficient and we can end up resuming the job before the progress was updated. This results in more spans being processed than expected.

This commit updates the test to instead check the contents of the job progress checkpoint and wait until the checkpoint contains all processed spans from before the pause.

Fixes: #153848

Release note: 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 #154758 is compliant with backport policy

Confidence: high
Backward compatible: true
Explanation: The pull request targets release-25.2 branch and modifies pkg/backup/backup_test.go, which is a test-only file (recognized by its _test.go suffix). According to the backport policy, changes that only affect non-production code, such as tests, are exempt from the standard backport policy criteria. Additionally, the PR body explicitly states 'Release justification: Test-only change.', which matches the exception criteria for test-only changes in the backport policy. Thus, the PR is compliant, as it accurately follows the exemption rules for test-only modifications.

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[cockroach-teamcity]

This change is