[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

tests/functional/fixtures/ext/foo/bar/language/en/permissions_foo.php

@@ -0,0 +1,6 @@
+<?php
+
+// Admin Permissions
+$lang = array_merge($lang, array(
+    'acl_u_foo'    => array('lang' => 'Can view foo', 'cat' => 'misc'),
+));