CI: increase timeout for Online DDL foreign key stress tests

[shlomi-noach]

Description

This PR increases the migration cut-over lock timeout in foreign key stress tests, to reduce flakiness caused by timeouts.

It also reduces migration check interval giving the Online DDL scheduler more opportunity to force cut-over the migration.

Related Issue(s)

No issue. Reducing CI flakiness.

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]

Review Checklist

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.56%. Comparing base (72149e2) to head (a2ed930).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16203      +/-   ##
==========================================
- Coverage   68.57%   68.56%   -0.01%     
==========================================
  Files        1544     1544              
  Lines      197863   197863              
==========================================
- Hits       135677   135658      -19     
- Misses      62186    62205      +19     

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