Fix test case names.

[codrut3]

This fixes a minor issue I found while working on #7291

The test case names don't match the method being tested. This is confusing because there is another method called merge_single_qubit_moments_to_phxz, but it's not the one tested here.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.68%. Comparing base (e27d883) to head (2ed73e7).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7406      +/-   ##
==========================================
- Coverage   98.68%   98.68%   -0.01%     
==========================================
  Files        1112     1112              
  Lines       97709    97709              
==========================================
- Hits        96427    96426       -1     
- Misses       1282     1283       +1     

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[pavoljuhas]

Thanks for cleaning this up!