Improve test suite

[btschwertfeger]

• Grouping tests into classes to share common variables and markers
• Use of session-scoped fixtures avoiding recreation for each test function
• Add GitHub Copilot instructions file

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.56%. Comparing base (cb80307) to head (c623b79).
⚠️ Report is 10 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #398      +/-   ##
==========================================
- Coverage   87.34%   83.56%   -3.78%     
==========================================
  Files          22       22              
  Lines        1959     1959              
==========================================
- Hits         1711     1637      -74     
- Misses        248      322      +74     

Flag	Coverage Δ
unittests	83.56% <ø> (-3.78%)	⬇️

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see 3 files with indirect coverage changes

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