A couple changes in Online DDL CI

[shlomi-noach]

Description

Submitting two changes in Online DDL CI for things I stumbled upon:

1. In "Online DDL flow" CI, used in Upgrade/Downgrade - wasn't waiting properly for migration to complete, escalating the termination logic. The existing logic still works, but ends up force-terminating the migration rather than waiting gracefully.
2. In Online DDL vrepl suite, we now recognize the $ONLINEDDL_SUITE_TEST_FILTER environment variable. If set, then we skip any tests not containing the value of $ONLINEDDL_SUITE_TEST_FILTER. If unset, then it is the empty string and trivially all test names contain the empty string.
   This is just in a way of making debugging/developing easier.

Related Issue(s)

None really.

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

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.59%. Comparing base (6e328de) to head (ca24159).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #16272   +/-   ##
=======================================
  Coverage   68.58%   68.59%           
=======================================
  Files        1544     1544           
  Lines      198018   198018           
=======================================
+ Hits       135817   135833   +16     
+ Misses      62201    62185   -16     

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