[ticket/11544] Add admin_login() method to 3.0 functional test case

This method is needed in order to be able to properly test acp functions.

PHPBB3-11544
Merge remote-tracking branch 'marc1706/ticket/11544' into develop-olympus

* marc1706/ticket/11544:
  [ticket/11544] Add admin_login() method to 3.0 functional test case