test: improving pool testing

[germa89]

Description

As the title.

Issue linked

Close #3735

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

Attention: Patch coverage is 7.14286% with 13 lines in your changes missing coverage. Please review.

Project coverage is 88.28%. Comparing base (b39d487) to head (0aeb1f3).
Report is 3 commits behind head on main.

❌ Your patch status has failed because the patch coverage (7.14%) is below the target coverage (90.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3736      +/-   ##
==========================================
+ Coverage   88.27%   88.28%   +0.01%     
==========================================
  Files         187      187              
  Lines       14705    14704       -1     
==========================================
+ Hits        12981    12982       +1     
+ Misses       1724     1722       -2     

[germa89]

@pyansys-ci-bot LGTM

[pyansys-ci-bot]

✅ Approving this PR because germa89 said so in here 😬

[germa89]

93513c0 reduced the teardown of the pool fixture from 20s to 5s by avoiding respawning.