[ticket/11792] Add functional test for var lang_set_ext of core.user_setup

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

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

@@ -0,0 +1,5 @@
+<?php
+
+$lang = array_merge($lang, array(
+	'SKIP'		=> 'Overwritten by foo',
+));