The 'loading' tests only worked when run with the other tests and
failed if you just ran the file by itself, because the skin was
loading in the 'joining' tests, but not here.
1. fix the 'Clean load' tests which had been broken by
https://github.com/matrix-org/matrix-react-sdk/pull/399: make sure we clear
localStorage between tests.
2. Test the session rehydration properly by setting the localStorage rather
than setting up the MatrixClientPeg before loading the app.
3. Add some tests for the auto-guest-registration flows.