CHORE: Use matplotlib Agg backend when testing

[SMoraisAnsys]

Multiple recent PRs struggled because figures get shown in the CICD and one would have to close them manually (which does not work through CICD). This leads to tests failing due to time out as no one closes the figures manually.

Here we use Agg backend which is a non-GUI backend and will avoid future PR to struggle with this issue.

For example, this happened in #4626
Also related to #4672

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 33.88%. Comparing base (a67460f) to head (04f6ebf).

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #4702       +/-   ##
===========================================
- Coverage   80.41%   33.88%   -46.53%     
===========================================
  Files         121      121               
  Lines       55100    55100               
===========================================
- Hits        44308    18671    -25637     
- Misses      10792    36429    +25637     

[SMoraisAnsys]

Since this changes might also impact the documentation, we might want to rename the environment variable a bit and see it's impact with the documentation.

[SMoraisAnsys]

Changes: I removed the use of the environment variable 'ON_CI' and added a fixture instead. This will ensure that the documentation is not impacted.

@Samuelopez-ansys If you want, we can update the test to remove the show=False argument. Now, even if show=True, the tests should be passing correctly. Let me know if you want to extend this PR with those changes.

[Samuelopez-ansys]

@SMoraisAnsys Yes please, remove the show=False, good job!

[MaxJPRey]

LGTM.

[SMoraisAnsys]

Removing show=False proved to have impact on the tests and led me to notice "misses" in our recent refactoring.
This PR should stay as a draft as long as #4723 is not merged.
Then tests should be updated accordingly.

[SMoraisAnsys]

I'll close this PR as adding those changes increases our tests by a huge overhead.
However, it was not entirely a waste of time as it allowed to catch #4730 !