add pytest-randomly and pytest-xdist to the testing env and CI

The addition of pytest-randomly IMO closes #330.

We may consider adding pytest-randomly as a testing dependency of Kwant so that tests also run in a deterministic way on people's machines.

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)