CI: rename TestSchemaChange to distinguish the tests

[shlomi-noach]

Description

Many (all) of the Online DDL CI tests used the test TestSchemaChange as the main entry point. They're in different packages and that's fine, but it made it more difficult for the maintainers to distinguish between the tests when looking at aggregated test failure reports.

In this PR we just change the name of the function per CI workflow.

Related Issue(s)

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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• Ensure that the Pull Request has a descriptive title.
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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.94%. Comparing base (01e3dd8) to head (99ca1ae).
Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #16694   +/-   ##
=======================================
  Coverage   68.93%   68.94%           
=======================================
  Files        1565     1565           
  Lines      201553   201556    +3     
=======================================
+ Hits       138944   138955   +11     
+ Misses      62609    62601    -8     

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