parallelize nox tests

[vchudnov-g]

Running nox (on all suites) on my local repo takes about 1hr. This obviously disrupts flow and prevents prompt feedback.

Let's try to speed up execution. Nox does not support background/parallel execution yet (see nox#88 and nox#198), so maybe we could have a top-level script that runs nox separately for each test suite and then possibly collects the results.

This will help development velocity and developer happiness.