Add integration tests

[farao]

Right now, the websocket handler is completely untested because we have only tests for the session (already a bit integration test because instead of mocking, this tests the workflow user sessions -> room -> user sessions) and more unit like tests for a room.

This would need:

1. choosing an elixir websocket library
2. connecting to the signaltower via network/websocket
3. sending json strings - maybe we just replicate the session tests in this form?

• should also tests functions that are only in the websocket handler and e.g. also that shutdown messages are sent