Replace pytest-qt with QtTest fatal reporting & reduce test data size of slow model test

[eivindjahren]

This ends up calling is_high_contrast_mode() and is_dark_mode() in a setting where there is no app so these need to check for that.

Also reduces the test data of the model test. This test took about 3 minutes in CI, now it takes less than a second.

• PR title captures the intent of the changes, and is fitting for release notes.
• Added appropriate release note label
• Commit history is consistent and clean, in line with the contribution guidelines.
• Make sure unit tests pass locally after every commit (git rebase -i main --exec 'just rapid-tests')

When applicable

• When there are user facing changes: Updated documentation
• New behavior or changes to existing untested code: Ensured that unit tests are added (See Ground Rules).
• Large PR: Prepare changes in small commits for more convenient review
• Bug fix: Add regression test for the bug
• Bug fix: Add backport label to latest release (format: 'backport release-branch-name')

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.66%. Comparing base (bd61fdf) to head (e3fcd77).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #12496      +/-   ##
==========================================
- Coverage   90.68%   90.66%   -0.03%     
==========================================
  Files         434      434              
  Lines       29569    29570       +1     
==========================================
- Hits        26816    26810       -6     
- Misses       2753     2760       +7     

Flag	Coverage Δ
cli-tests	37.71% <0.00%> (+<0.01%)	⬆️
gui-tests	68.95% <100.00%> (-0.06%)	⬇️
performance-and-unit-tests	74.30% <100.00%> (+0.03%)	⬆️
test	38.43% <0.00%> (-0.01%)	⬇️

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[codspeed-hq]

CodSpeed Performance Report

Merging #12496 will not alter performance

_{Comparing eivindjahren:use_same_qt_model_testing_mode (e3fcd77) with main (8fc6e0e)}

Summary

✅ 22 untouched

[andreas-el]

💯