Faster room joins: Add happy path tests for device list tracking (#475)

This change tests device list tracking for pre-existing and new members
in partial state rooms.

In particular, we test that:
 * remote device lists are correctly cached or not cached
 * local users are told about potential device list changes in `/sync`'s
   `device_lists.changed/left`
 * local users are told about potential device list changes in `/keys/changes`