Speed up tests with pytest-xdist

[simonw]

I think I can get this working for almost every test, then use the pattern in pytest-dev/pytest-xdist#385 (comment) to opt specific tests out of being run in parallel.

[simonw]

On my Mac pytest-xdist ran the test suite (minus two tests) in 59s, as opposed to 2m23s without xdist.

[simonw]

Looks like -n auto only detected two cores on GitHub Actions: https://github.com/simonw/datasette/runs/2257597137?check_suite_focus=true

============================= test session starts ==============================
platform linux -- Python 3.7.10, pytest-6.2.2, py-1.10.0, pluggy-0.13.1
SQLite: 3.31.1
rootdir: /home/runner/work/datasette/datasette, configfile: pytest.ini
plugins: xdist-2.2.1, timeout-1.4.2, forked-1.3.0, asyncio-0.14.0
gw0 I / gw1 I
gw0 [878] / gw1 [878]

[simonw]

https://github.com/simonw/datasette/actions/runs/713207828 ran with pytest-xdist in 4m22s:

Here's the test suite running on regular pytest in 5m13s:

Not a huge speed-up because there are only 2 available cores in the GitHub Actions environment, but still worthwhile - especially since this lets people run in parallel on their own laptops.