[ticket/11458] Search for permission language files in extensions

Extensions that add new permission masks only need to add a permission file in
the language folder of the extension. The file must start with 'permissions_'
eg 'permissions_blog.php'. The permission language file will be automatically
included when viewing/setting permissions.

PHPBB3-11458
[ticket/11458] We still auto add language files from the mods and acp
folders in the language directory, so we revert some changes here

PHPBB3-11458
[ticket/11458] Add functional test

Since there is no test method to include extension language files, a
functional test seems more appropriate.
We add a permission mask 'acl_u_foo' with translation found in extenion
'bar' and confirm that the permission language file 'permissions_foo.php'
from 'bar' was added by asserting that 'Can view foo' exists when viewing
user permissions in acp

PHPBB3-11458
[ticket/11458] Fix test

PHPBB-11458
[ticket/11458] Fix functional test again

Add 'u_foo' to phpbb_acl_options
Make sure extension and fixtures are added before running test

PHPBB3-11458
[ticket/11458] Use helper to create/move/delete directories/files

PHPBB3-11458
[ticket/11458] Use finder to get all permission language files

PHPBB3-11458
[ticket/11458] Update functional test

Show that the phpbb permission lanuage file is being included

PHPBB3-11458
Merge remote-tracking branch 'remotes/blitze/ticket/11458' into develop