refactor: moving tests to classes to avoid repeated fixtures execution

[germa89]

Description

Moving the tests to classes to speed up testing.

Performance

Before

  Time (mean ± σ):     42.118 s ±  5.528 s    [User: 8.025 s, System: 3.524 s]
  Range (min … max):   35.268 s … 53.661 s    10 runs

After

  Time (mean ± σ):     28.822 s ±  7.450 s    [User: 6.949 s, System: 3.077 s]
  Range (min … max):   22.796 s … 43.364 s    10 runs

Issue linked

NA

Checklist

• I have visited and read Develop PyMAPDL.
• I have tested my changes locally
• I have added necessary documentation or updated existing documentation.
• I have followed the PyMAPDL coding style guidelines.
• I have added appropriate unit tests that also ensure the minimal coverage criteria.
• I have reviewed my changes before submitting this pull request.
• I have linked the issue or issues that are solved to the PR if any.
• I have assigned this PR to myself.
• I have marked this PR as draft if it is not ready to be reviewed yet.
• I have made sure that the title of my PR follows Commit naming conventions (e.g. feat: adding new MAPDL command)

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.60%. Comparing base (e63570e) to head (ce97c89).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3523      +/-   ##
==========================================
+ Coverage   82.40%   85.60%   +3.19%     
==========================================
  Files          58       58              
  Lines       10017    12348    +2331     
==========================================
+ Hits         8255    10571    +2316     
- Misses       1762     1777      +15     

[germa89]

@pyansys-ci-bot LGTM.

[pyansys-ci-bot]

✅ Approving this PR because germa89 said so in here 😬