refactor: moving tests to a class and adding delete method.

[germa89]

Description

Moving pool tests to a class. The idea is to make sure we leave no instances behind.

Issue linked

Related to #3254

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 83.06%. Comparing base (03f722e) to head (9dfd750).
Report is 4 commits behind head on main.

❗ There is a different number of reports uploaded between BASE (03f722e) and HEAD (9dfd750). Click for more details.

HEAD has 84 uploads less than BASE

Flag	BASE (03f722e)	HEAD (9dfd750)
dmp	17	6
minimal	7	3
ubuntu	21	9
local	14	6
non-student	16	0
smp	8	3
remote	11	3
v23.2-ubuntu	3	0
v24.1-ubuntu	3	0
centos	4	0
v24.2.0	1	0
v24.2-ubuntu	3	0
latest-ubuntu	3	0
v24.1.0	1	0
v25.1.0	1	0
v23.1.0	1	0

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3258      +/-   ##
==========================================
- Coverage   88.22%   83.06%   -5.16%     
==========================================
  Files          53       55       +2     
  Lines        9670     9753      +83     
==========================================
- Hits         8531     8101     -430     
- Misses       1139     1652     +513     

[germa89]

@clatapie please review :)

[clatapie]

Looks good to me!