Remove usage of additional test package

[dbussink]

We already extensively use testify, so let's make sure we use that everywhere and not add an additional testing dependency.

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
• 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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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6c4c5fa) 47.34% compared to head (27e5ee2) 47.34%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #15007   +/-   ##
=======================================
  Coverage   47.34%   47.34%           
=======================================
  Files        1144     1144           
  Lines      238997   238997           
=======================================
+ Hits       113150   113154    +4     
+ Misses     117249   117244    -5     
- Partials     8598     8599    +1     

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