Add test script

[florimondmanca]

Prompted by #399 (comment)

Running

$ scripts/test

runs the test suite and runs the check session.

I made it so that the test suite only for the current Python version is run (e.g. 3.7), but we could also run $ nox -s test so that all available Python versions are tested (e.g. all of 3.6, 3.7 and 3.8 if they're detected as available by nox). This is mostly because running tests against all Python versions can get very long, esp. because virtualenvs aren't reused. Thoughts?

[florimondmanca]

I'm also wondering — what should we do about the contributing docs, since they currently recommend using $ nox -s test?

[pquentin]

Would it help to configure nox to reuse virtual environments? There are various places where it can be done: https://nox.thea.codes/en/stable/config.html

[florimondmanca]

@pquentin We actually reuse virtualenvs in all other nox sessions, but not in test. I think reusing would be OK in that case too though. We can always --no-reuse-existing-virtualenvs on the command line. I'll submit a separate PR for that. :)