Add test for vindexes in atomic transactions package

[GuptaManan100]

Description

Like the last PR, this one too add tests for atomic distributed transactions. This time we're adding tests for vindexes. Like last time, we didn't expect anything to fail, but its always nice to have tests to verify the assumptions.

Related Issue(s)

• Fixes RFC: Atomic Distributed Transactions #16245

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

Deployment Notes

[vitess-bot]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.41%. Comparing base (0726ea6) to head (886c7c3).
Report is 13 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #17308   +/-   ##
=======================================
  Coverage   67.40%   67.41%           
=======================================
  Files        1574     1574           
  Lines      253221   253277   +56     
=======================================
+ Hits       170690   170735   +45     
- Misses      82531    82542   +11     

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