schemadiff: adding charset/collation tests

[shlomi-noach]

Description

Just adding a couple test cases. No particular issue, though this is inspired by #15859

Related Issue(s)

#15859

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

Hello reviewers! 👋 Please follow this checklist when reviewing this Pull Request.

General

• Ensure that the Pull Request has a descriptive title.
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Tests

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.42%. Comparing base (ab525d9) to head (fc29830).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #15872   +/-   ##
=======================================
  Coverage   68.42%   68.42%           
=======================================
  Files        1559     1559           
  Lines      196537   196537           
=======================================
  Hits       134484   134484           
  Misses      62053    62053           

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