Faster room joins: Add happy path tests for outgoing device list updates (#473)

This change tests that:
 * device list updates are sent to the servers listed in the
   `/send_join` response
 * device list updates are sent to servers that join after we start the
   partial state join
 * device list updates stop being sent to servers listed in the
   `/send_join` response that are no longer in the room once the partial
   state join completes.

Author: squahtx