Consider using Jest instead of Ava

[okonet]

Ava is awesome but I think Jest offers so much more out of the box. Particularly I wanted to write a test for a fix of #632 but this seems not possible without pulling in at least 2 additional dependencies for 1. mocking and stubbing (sinon?) and 2. for hijacking the node require in order to mock the Jest APIs. With Jest that would not require any additional dependencies.

Thoughts?

[eliperelman]

I am totally open to using whatever improves our testing situation. I think at the time I used AVA because it was fast, and Jest wasn't really a thing I knew could be used outside React. But yeah, I'm totally open to it.

[eliperelman]

As an aside, AVA now supports snapshot testing.