Convert tests to TypeScript

[from-the-river-to-the-sea]

Our tests are in raw JS right now, which means they aren't really testing our types, and they're a bit harder to write due to not having TS autocomplete.

We should re-write them in TS, and also ensure that we are applying the obs-websocket-js typedefs to our mock-obs-websocket-js, to ensure that our mock is the right shape.

See https://github.com/Tespa/hearthstone-parser for an example of how I like to handle tests in TypeScript (although that repo uses Mocha, not Ava, so it's not a 1:1 example).