test: Cleaner plan tests output

[systay]

Description

Test infrastructure improvement.

This PR changes the plan test output to be easier to work with. It will leave a single directory after running the plan tests, and this directory will only contain the failed files. When running tests, the expectation directory will be emptied.

I suggest we backport this just to make it easier to work with backports. If there are any issues with the backports, they can just be abandoned.

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

[vitess-bot]

Review Checklist

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• vtctl command output order should be stable and awk-able.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.45%. Comparing base (0353ad4) to head (491f2a7).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15922      +/-   ##
==========================================
- Coverage   68.45%   68.45%   -0.01%     
==========================================
  Files        1559     1559              
  Lines      196825   196832       +7     
==========================================
+ Hits       134736   134738       +2     
- Misses      62089    62094       +5     

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