[release-19.0] onlineddl_scheduler test: fix flakiness in artifact cleanup test (#15396)

[vitess-bot]

Description

This is a backport of #15396

[vitess-bot]

Review Checklist

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.42%. Comparing base (a327e2b) to head (a0c8e6c).
Report is 1 commits behind head on release-19.0.

Additional details and impacted files

@@               Coverage Diff                @@
##           release-19.0   #15399      +/-   ##
================================================
- Coverage         67.43%   67.42%   -0.01%     
================================================
  Files              1560     1560              
  Lines            192642   192642              
================================================
- Hits             129906   129888      -18     
- Misses            62736    62754      +18     

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[frouioui]

We are not force-merging this for v19 GA as this is only changing test code.