Online DDL: fix flaky onlineddl_scheduler CI test

[shlomi-noach]

Description

This PR fixes a flakiness in onlineddl_scheduler CI. It seems to have been lurking for quite a while, but I only encountered it severely in #15988, though after a long investigation, I found it's unrelated to the throttler.

It's a really simple race condition between VReplication and a SELECT FOR UPDATE query the test runs (validating the behavior of FORCE_CUTOVER). The race is that the test can issue said select query before VReplication has even had the chance to start running, in which case VReplication never gets to the point of cutting over, in which case the migration cannot cut-over. This is unrealistic (or rather, not interesting) in production, and not the purpose of the test.

The fix is to ensure VReplication has had chance to run before validating the cutover behavior.

On top of that change, we also use "--migration_check_interval", "5s", which is standard across all Online DDL CI jobs, and overall reduces runtime of this CI job.

Related Issue(s)

No related issue. See for example this CI failure.

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

[vitess-bot]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.26%. Comparing base (d95ce48) to head (dca9836).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16011      +/-   ##
==========================================
- Coverage   68.27%   68.26%   -0.02%     
==========================================
  Files        1562     1562              
  Lines      197319   197319              
==========================================
- Hits       134714   134690      -24     
- Misses      62605    62629      +24     

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