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)