tests: add tests for go/mysql/format

[Maniktherana]

Description

Achieves 100% coverage for go/mysql/format

Related Issue(s)

#14931

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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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (c1f9c80) 47.26% compared to head (aa77c64) 47.29%.
Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14981      +/-   ##
==========================================
+ Coverage   47.26%   47.29%   +0.03%     
==========================================
  Files        1138     1140       +2     
  Lines      238842   238862      +20     
==========================================
+ Hits       112880   112963      +83     
+ Misses     117368   117307      -61     
+ Partials     8594     8592       -2     

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[dbussink]

@beingnoble03 Found a bug in this package, see #14972. So they are going to fix that and likely also add tests, superseding this PR. It's a good reminder to always also make sure to try and break code when you add tests, not just test the happy paths.