onlineddl_scheduler test: fix flakiness in artifact cleanup test

[shlomi-noach]

Description

We've seen flakiness in onlineddl_scheduler test, TestSchemaChange/scheduler/Cleanup_artifacts/validate_artifact_exists. The test run an open ended (--postpone-completion) migration, sets a very low artifact retention, and looks to see that its artifacts exist while the migration is running. Artifacts should only be deleted when the migration completes, fails, or gets cancelled, so this is intended to validate we do not cleanup prematurely.

However, it's possible that the artifact is not yet created, and that's the race condition. The fact the test is in running state, does not mean it's reached the part where the shadow table gets created.

The fix to the flakiness is to wait until the migration is ready_to_complete. This guarantees that the artifact (shadow table) is already created.

Related Issue(s)

https://github.com/planetscale/vitess-private/actions/runs/8112537427/job/22173985451?pr=4573

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.44%. Comparing base (696fe0e) to head (77ae980).
Report is 68 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15396      +/-   ##
==========================================
- Coverage   67.41%   65.44%   -1.98%     
==========================================
  Files        1560     1562       +2     
  Lines      192752   193911    +1159     
==========================================
- Hits       129952   126911    -3041     
- Misses      62800    67000    +4200     

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[deepthi]

I thought we should backport at least to v19. We can consider backporting beyond that if the test shows flakiness in older branches.