[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