Fix test so that we don't mangle origin-eventID or origin-sender relationship when breaking sig (otherwise the test triggers *other* problems) (#963)

Author: neilalexander

tests/50federation/30room-join.pl

@@ -1027,7 +1027,7 @@ sub assert_is_valid_pdu {
       );
 
       # Modify the event (after the signature was generated) to invalidate the signature.
-      $event->{origin} = "other-server:12345";
+      $event->{state_key} = "something";
 
       my $await_request_send_join;