[release-21.0] Flaky test fix: TestMoveTablesSharded and TestMoveTablesUnsharded (#17343)

[vitess-bot]

Description

This is a backport of #17343

[vitess-bot]

Review Checklist

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General

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.50%. Comparing base (656f35c) to head (bd34a19).
Report is 8 commits behind head on release-21.0.

Additional details and impacted files

@@               Coverage Diff                @@
##           release-21.0   #17363      +/-   ##
================================================
+ Coverage         67.49%   67.50%   +0.01%     
================================================
  Files              1570     1570              
  Lines            251411   251411              
================================================
+ Hits             169680   169727      +47     
+ Misses            81731    81684      -47     

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