tests: fix pyside6 segfault in cmap combo tests

[tlambert03]

fixes #318

something happened in pyside 6.10 that prevents standard mocking patterns (which we were using in our tests).

This attempts to workaround the regression with a different pattern

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.40%. Comparing base (5335004) to head (d7cf864).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #319      +/-   ##
==========================================
- Coverage   86.40%   86.40%   -0.01%     
==========================================
  Files          49       49              
  Lines        3862     3861       -1     
==========================================
- Hits         3337     3336       -1     
  Misses        525      525              

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