test: diffmap plot coverage

[joncarse]

• Closes tests for sc.pl.diffmap and sc.pl.rank_genes_groups #3787
• Tests included

• Release notes not necessary because: no release notes for test-only PRs

This PR covers diffmap. ranked_genes_groups seems to have sufficient coverage in main, maybe OP can give more details into what isn't covered?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.22%. Comparing base (3d01c78) to head (460edb7).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4273      +/-   ##
==========================================
+ Coverage   79.21%   79.22%   +0.01%     
==========================================
  Files         128      128              
  Lines       13446    13446              
==========================================
+ Hits        10651    10653       +2     
+ Misses       2795     2793       -2     

Flag	Coverage Δ
hatch-test.low-vers	77.47% <ø> (+0.01%)	⬆️
hatch-test.pre	79.10% <ø> (+0.01%)	⬆️

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see 1 file with indirect coverage changes

[flying-sheep]

thank you, this is wonderful!