test: Add missing tests for go/mysql/datetime

[beingnoble03]

Description

This PR adds missing tests for go/mysql/datetime

Related Issue(s)

• Improve Unit Test Coverage #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

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

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

• Bug fixes should have at least one unit or end-to-end test, enhancement and new features should have a sufficient number of tests.

Documentation

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• There should be some code comments as to why things are implemented the way they are.
• There should be a comment at the top of each new or modified test to explain what the test does.

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Backward compatibility

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• If a flag is removed, then it should also be removed from vitess-operator and arewefastyet, if used there.
• 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 65.72%. Comparing base (d5bd597) to head (99a9a32).
Report is 20 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15501      +/-   ##
==========================================
+ Coverage   65.64%   65.72%   +0.08%     
==========================================
  Files        1563     1562       -1     
  Lines      194389   194441      +52     
==========================================
+ Hits       127602   127803     +201     
+ Misses      66787    66638     -149     

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[GuptaManan100]

Rest LGTM!