Switch from Mocha to Jest for "Better" testing

[KamalAman]

I find the Jest runtime more enjoyable than mocha for a few reasons:

Assertions and reporting
Mocking (which much simplify some of the current unit tests)
Parallel processing

Would do accept a PR that will migrate?

[rolftimmermans]

Sure! I'm not bound to any particular testing framework, so whatever works best is fine.

The only thing I personally dislike is a DSL for assertions like expect(a + b).not.toBe(0) which IMHO obscure what is being asserted. I like chai's assertion library personally... 🤷‍♀

[rolftimmermans]

I'd be happy to accept PRs that change the test suite for the better, but closing this for now since changing test framework is not a goal in itself.