Change the testing framework to jest

[brianmhunt]

Having evaluated the options and @caseyWebb 's comment suggesting Jest, I think it's a great option, for a couple reasons:

1. It handles async code smartly, eliminating pesky race conditions
2. jest-codemods does much migration automatically, particularly easily for Mocha
3. Once homogenized we can have a single test-runner, which should dramatically speed things up
4. It looks to be based on jasmine, so a bunch of legacy tests should be straightforward

Thanks @caseyWebb for the suggestion to look into Jest. It looks to be a smart move.