CI: wait-for rather than 'assume' in Online DDL flow

[shlomi-noach]

Description

Quick followup to #16017: there was an assumption that by the end of 3sec into the migration, this would hold:

assert.GreaterOrEqual(t, appliedDMLEnd-appliedDMLStart, int64(maxTableRows))

Normally that's correct, the number is actually 3 times larger than int64(maxTableRows) on GitHub CI. And yet, this is a source for flakiness on smaller testing runners.

This PR opts to actually wait for that value, with a max timeout of 1min, which is currently 60 times more than needed in GitHub CI, and still with high margins in slower runners.

Related Issue(s)

#16017

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
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• Tests were added or are not required
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• 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 68.55%. Comparing base (6f85089) to head (02743d1).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16210      +/-   ##
==========================================
- Coverage   68.58%   68.55%   -0.04%     
==========================================
  Files        1544     1544              
  Lines      197873   197873              
==========================================
- Hits       135716   135652      -64     
- Misses      62157    62221      +64     

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