Adding unit testing section

[clatapie]

Closes #1536

[github-actions]

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[codecov]

Codecov Report

Merging #1621 (6d23a75) into main (8850205) will decrease coverage by 0.09%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1621      +/-   ##
==========================================
- Coverage   82.84%   82.74%   -0.10%     
==========================================
  Files          44       44              
  Lines        7151     7151              
==========================================
- Hits         5924     5917       -7     
- Misses       1227     1234       +7     

[RobPasMue]

It's looking good. Can't add more than @germa89's comments =)

[RobPasMue]

LGTM in general! Some minor comments. Leaving final review to @germa89 (since he is the PyMAPDL boss =P)

[germa89]

Hi Camile

I have left some comments in the PR, most of them are grammar based. Others are based on the following suggestions:

• Add all the links to links.rst and then reference it from there as explained in Adding links.rst to docs #1506
• Explain a bit more about how to couple your tests with the existing ones. i.e. explain what are fixtures.
• Reference any document, text, image, or whatever thing you do copy from somewhere else.

With the proposed changes, you will have to add these links to links.rst:

.. _pytest: pytest.org
.. _pymapdl_tests: https://github.com/pyansys/pymapdl/tree/main/tests
.. _codecov: https://github.com/codecov
.. _pytest_usage: https://docs.pytest.org/en/7.2.x/how-to/usage.html#specifying-which-tests-to-run
.. _article_good_unit_test: https://stackoverflow.com/questions/61400/what-makes-a-good-unit-test 
.. _mapdl_fixture: https://github.com/pyansys/pymapdl/blob/fb5fb8b6201253f1bd56bdabee60a29abee8c7d8/tests/conftest.py#L254
.. _pymapdl_test_math: https://github.com/pyansys/pymapdl/blob/main/tests/test_math.py
.. _pymapdl_user_guide_math: https://mapdl.docs.pyansys.com/user_guide/math.html

[germa89]

Pinging @maromer0 for learning purposes.

Please Manu have a look at this PR and start to get familiarized with how the documentation is written using RST.

[RobPasMue]

@germa89 you have to approve so that we can merge =)

[germa89]

LGTM.