endtoend: better error reporting in Online DDL MySQL execution tests

[shlomi-noach]

Description

Some Online DDL tests run the mysql command line. When that execution fails, all we got was "exit status 1". We now ensure to capture standard error in a temp file, whose contents we read and add to the error message.

Related Issue(s)

None.

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
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• Tests were added or are not required
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• 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 68.95%. Comparing base (fd18ae4) to head (666e231).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16786      +/-   ##
==========================================
+ Coverage   68.93%   68.95%   +0.01%     
==========================================
  Files        1565     1565              
  Lines      201741   201741              
==========================================
+ Hits       139076   139108      +32     
+ Misses      62665    62633      -32     

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