Use ava for tests

[SimenB]

I much prefer ava over mocha, regardless of assertion library used with mocha.

Unless @wojciechczerniak objects, I'll refactor after we whip the tests into shape (#351). Not a blocker for 2.0 of any kind, nor a priority. But something I want to do

[wojciechczerniak]

Cool. I'm always open to learn something new 😉

[SimenB]

As we use assert already, the assertions themselves won't be too different (t.true(something.deep === 'five')) instead of assert.equal(something.deep, 'five'). Diff is that we won't have nested describes, and we have to be careful about state (as ava executes in parallel)