Do a clearer join -> join transition for restricted rooms tests. (#208)

This exposes a bug in Synapse where a join -> join transition
could cause a bogus "join_authorised_via_users_server" field
to be included in the event.

tests/restricted_rooms_test.go

@@ -82,7 +82,21 @@ func checkRestrictedRoom(t *testing.T, alice *client.CSAPI, bob *client.CSAPI, a
 	bob.JoinRoom(t, room, []string{"hs1"})
 
 	// Joining the same room again should work fine (e.g. to change your display name).
-	bob.JoinRoom(t, room, []string{"hs1"})
+	bob.SendEventSynced(
+		t,
+		room,
+		b.Event{
+			Type:     "m.room.member",
+			Sender:   bob.UserID,
+			StateKey: &bob.UserID,
+			Content: map[string]interface{}{
+				"membership":  "join",
+				"displayname": "Bobby",
+				// This should be ignored since this is a join -> join transition.
+				"join_authorised_via_users_server": "unused",
+			},
+		},
+	)
 
 	// Leaving the room works and the user is unable to re-join.
 	bob.LeaveRoom(t, room)