Fix "inbound federation redacts events from erased users"

Ensure the HS under test remains in the room so we can query the event
which should be redacted.

Fixes #1203.

tests/50federation/32room-getevent.pl

@@ -45,12 +45,22 @@
  requires => [
  $main::OUTBOUND_CLIENT,
  federated_rooms_fixture(),
+ local_user_fixture(),
  ],
 
+ # On the homeserver under test, have a user `creator` create a public room.
+ # A second user `remaining_user` on the same HS joins too.
+ # Sytest acts as a second homeserver and join a user (`user_id`) to that room.
+ # The `creator` sends a message, then requests deactivation with erasure.
+ # The homeserver under test should redact that event. Sytest confirms this by
+ # re-requesting that event. (The `remaining_user` ensures that the homeserver under
+ # test is still in the room.)
+
  do => sub {
- my ( $outbound_client, $creator, $user_id, $room ) = @_;
+ my ( $outbound_client, $creator, $user_id, $room, $remaining_user ) = @_;
  my $first_home_server = $creator->server_name;
  my $room_id = $room->room_id;
+ matrix_join_room($remaining_user, $room_id);
 
  my $message_id;
 
@@ -79,7 +89,7 @@
  # Check that the content is right
  assert_eq( $event->{content}->{body}, "body1" );
 
- # now do the erasure
+ # The `creator` requests that their account be deactivated.
  matrix_deactivate_account( $creator, erase => JSON::true );
  })->then( sub {
  # re-fetch the event and check that it is redacted.