[JOSS Review] Test requirements

This issue is part of the JOSS Review in openjournals/joss-reviews#3419.

I would like to execute the tests in mpi4jax, which use pytest, on my desktop, but there are no instructions nor requirements available for performing these tests. Could a list of requirements be added, either as a file that can be installed with pip install -r requirements_test.txt, or simply adding it to the setup() function in the setup.py file?

[PhilipVinc]

@1313e thanks for your review.

@dionhaefner is it ok if we go with adding an extras_require={'dev':[pytest, black, precommit...]} so that if one does pip install mpi4jax[dev] then everything will work out of the box?

PyTest is already set up to run the tests by simply invoking pytest so we might need to write in the documentation somewhere that you are supposed to invoke pytest && mpirun -np 2 pytest

[1313e]

Might be a good idea to install the package using the described way from the online documentation in GitHub Actions, to make sure that there is a minimal difference between the user way and automated way of testing.