TST: Add test for retrieving P image with alpha mask

[stefan6419846]

No description provided.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.16%. Comparing base (bd433f7) to head (7aba376).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3525      +/-   ##
==========================================
+ Coverage   97.14%   97.16%   +0.02%     
==========================================
  Files          57       57              
  Lines        9807     9807              
  Branches     1780     1780              
==========================================
+ Hits         9527     9529       +2     
+ Misses        168      167       -1     
+ Partials      112      111       -1     

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