Add pytest-randomly and pytest-xdist to the testing environment and CI builds

The addition of pytest-randomly IMO closes #330.

pytest-xdist should make the tests run faster (on my laptop running with pytest -n auto goes from 54s to 38s). We may need to tweak -n auto to something else, to avoid all the cores on the machine being hogged (given that we simultaneously run the tests in the different testing envs)