Don't send events all the way via the mock HTTP backend: we're not
trying to test that here. This meant we weren't actually getting
into the right state because the request to send the invite never
actually returned. Now this works, we need to clear the invite timer
otherwise jest has a timer hanging around at the end of the test
(plus we should be doing it anyway).
Typescript tests basically just appear to work, apart from needing
the jest types imported so the typescript checker knows what's what.
DConvert the webrtc test to typescript, which actually mostly just
serves to point out that we're not mocking the whole of `document`,
but oh well.
