Convert testfiles from unittest to pytests

[PicoCentauri]

We are using pytest for testing but still the testsuite uses the unittest style. Moving it to pytest makes sense also in the light of fixing #279 as handling warnings is nice in pytest.

As this is a very monotonic job, I mostly vibecoded this with Copilot based on Sonnet 4.5. To help Copilot, I created an instruction file with some useful tricks on how to work on the repo.

Besides, I also cleaned up the tests:

• use np.testing instead of concatenating assert and numpy functions
• if we use pytest.raises() we always match against the specific error message to avoid test passing if a wrong error is raised.

I don't expect a line by line review for this PR. What I did and I also would do for a review is to check example conversations of some tests. Copilot had especially problems with test_kernel_pcovc.py and test_pcovc.py...

[cajchristian]

These changes look good. I also gave the PCovC/R and KCPovC/R tests a deeper look through, and everything looks great. Can you also update contributing.rst?

[cajchristian]

Thanks!