run tests locally with pytest

[ersisimou]

Hi,

In order to run tests locally with pytest, the pytest, pytest-xdist, pytest-memray, pytest-cov are currently included in setup.cfg.

However, it is not possible to install pytest-memray on a Mac. So, we might want to remove that.

Also (very minor) we might want to change the path in contributing.md for running tests, to be a valid path to a test in ott