Tests: replace mocked Authenticator with the real thing (#11913)

If we prepopulate the test homeserver with a key for a remote homeserver, we
can make federation requests to it without having to stub out the
authenticator. This has two advantages:

 * means that what we are testing is closer to reality (ie, we now have
   complete tests for the incoming-request-authorisation flow)

 * some tests require that other objects be signed by the remote server (eg,
   the event in `/send_join`), and doing that would require a whole separate
   set of mocking out. It's much simpler just to use real keys.