[chore] Add merge_group trigger to test workflow

[mx-psi]

Adds merge_group to add support for a merge queue on this repository:

https://docs.github.com/en/repositories/configuring-branches-and-merges-in-your-repository/configuring-pull-request-merges/managing-a-merge-queue#triggering-merge-group-checks-with-github-actions

I only added it on the workflow that has required jobs

This assumes we want a merge queue. If you think that's a bad idea please complain!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 64.08%. Comparing base (0939b56) to head (4ee725f).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1683   +/-   ##
=======================================
  Coverage   64.08%   64.08%           
=======================================
  Files          68       68           
  Lines        3447     3447           
=======================================
  Hits         2209     2209           
  Misses        997      997           
  Partials      241      241           

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