[ticket/11465] Add disabled ext to allow proper testing of get_module_infos()

This will now also enable us to test the $use_all_available parameter of
get_module_infos(), which will not only return the module infos for enabled
extensions but also those from disabled extensions.

PHPBB3-11465

tests/extension/ext/barfoo/ext.php

@@ -0,0 +1,5 @@
+<?php
+
+class phpbb_ext_barfoo_ext extends phpbb_extension_base
+{
+}