Fix "... non-joined user can call /events on world_readable room" fla…

…kes (#1346)

The test was flaking because it was sometimes waiting for events to come
down /events after they had already appeared in the stream.

Signed-off-by: Sean Quah <seanq@matrix.org>

tests/30rooms/30history-visibility.pl

@@ -147,6 +147,13 @@ sub test_history_visibility
             matrix_set_room_history_visibility_synced( $user, $room_id, "world_readable" );
          })->then( sub {
             matrix_initialsync_room( $nonjoined_user, $room_id )
+         })->then( sub {
+            # Get a token and configure the waits for events below to start from it.
+            matrix_get_events( $nonjoined_user, room_id => $room_id, timeout => 0 )
+            ->then( sub {
+               my ( $body ) = @_;
+               $nonjoined_user->eventstream_token = $body->{end};
+            });
          })->then( sub {
             Future->needs_all(
                matrix_send_room_text_message_synced( $user, $room_id, body => "mice" )