[ticket/11744] Helper function to assert notifications in base test

This also fixes tests

PHPBB3-11744
Merge branch 'ticket/11744' of github.com:EXreaction/phpbb3 into ticket/11745

* 'ticket/11744' of github.com:EXreaction/phpbb3:
  [ticket/11744] Helper function to assert notifications in base test
[ticket/11745] Add test for group request approved

PHPBB3-11745