REFACTOR: Test architecture and CICD

[SMoraisAnsys]

Follow up of #5313

Changes:

• Split tests between system and unit tests
• Update CICD to use add unit tests job; use common pytest options (impact: durations is set to 25 everywhere)
• Update configuration related to pytest / testing to match tests folder architecture
• Remove _unittest_ironpython folder

Note: the coverage of the tests is no longer performed by default when running the tests of pyaedt.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.34%. Comparing base (6462a41) to head (fc98d98).
Report is 14 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5351      +/-   ##
==========================================
+ Coverage   84.32%   84.34%   +0.01%     
==========================================
  Files         140      140              
  Lines       58460    58460              
==========================================
+ Hits        49298    49307       +9     
+ Misses       9162     9153       -9     

[Samuelopez-ansys]

Great job!