Add ability to run tests in parallel

[giampaolo]

We can do this now that we use pytest.

[giampaolo]

Done in e23b8ee1. The speedup with 14 cores is impressive.

$ make test
...
=== 705 passed, 91 skipped, 1 warning in 12.99s ===

$ make test-parallel
...
==== 705 passed, 91 skipped, 1 warning in 2.55s ===