Switch to ava

[tmcw]

Tests take 2 minutes: I'd love for them to finish in under a minute.

This is a theory ticket: if ava doesn't work well, I'll close without switching.

[tmcw]

Notes:

• ava doesn't support nested tests
• t.end and t.done will throw if the test doesn't take a callback

assertions are very fancy. too fancy. weirdly fancy.

[tmcw]

Nope: ava is great for other things, but isn't right for documentation.js.

[arv]

I just started using jest on a project and I like that dependency management makes it so that it only runs tests that are affected by your changes.

[tmcw]

Good idea! Jest, I think, also runs test files in parallel. Trying it out in #667!