chore: use interface instead of struct for tests

[systay]

Description

We don't want to be tied down to using the testing package when using MySQLCompare, here we introduce an interface to make it possible to have other implementations.

[vitess-bot]

Review Checklist

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General

• Ensure that the Pull Request has a descriptive title.
• Ensure there is a link to an issue (except for internal cleanup and flaky test fixes), new features should have an RFC that documents use cases and test cases.

Tests

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Documentation

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Backward compatibility

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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 65.77%. Comparing base (6976baa) to head (d6ef271).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15581      +/-   ##
==========================================
+ Coverage   65.75%   65.77%   +0.01%     
==========================================
  Files        1561     1561              
  Lines      194830   194880      +50     
==========================================
+ Hits       128110   128175      +65     
+ Misses      66720    66705      -15     

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