[ticket/11538] Add admin as admins leader and moderator in memberlist_test

Removing the admin user from the admin and moderator group in memberlist_test
potentially breaks other tests, i.e. the ucp groups test.

PHPBB3-11538
Merge remote-tracking branch 'marc1706/ticket/11538' into develop

* marc1706/ticket/11538:
  [ticket/11538] Add admin as admins leader and moderator in memberlist_test