To ensure that the new lang_set_ext variable available with the
core.user_setup event works properly, a functional test was added. It
overwrites the value of the 'SKIP' language key, which is assumed to remain
in use for some time to come.
PHPBB3-11792
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
