When we no longer share any rooms with a given user, the server will stop
sending us updates on their device list, and will (once synapse is updated)
send us a notification of that fact via the 'left' field in the device_lists
field in /sync, or the response from /keys/changes.
There's a fuzzy line between the megolm tests and the devicelist ones, but
since I want to add more tests for devicelists, we might as well put the ones
which are definitely about devicelists in their own file