Consider running tests in ava for a speed boost

[kumar303]

Is this a feature request or a bug?

Feature request

What is the current behavior?

The tests are slow.

What is the expected or desired behavior?

The tests should be fast!

I was looking at ava because it promises a speed-up for I/O heavy tests (like the web-ext test suite). I fiddled with it a bit but I think it requires moving web-ext to babel 7 first. Switching to ava might be better than Jest (https://github.com/mozilla/web-ext/issues/1279) but I don't know.

Note that ava 1.0.0 is about to come out. They have just published 1.0.0-rc.1.