PHPBB3-10586 added a reference to phpbb_functional_path in the $config array, which was never created. It appears that this functionality was moved into ticket PHPBB3-10706 (which was denied), but not all of it was removed from 10586's pull request, so it was merged in. In /tests/functional/extension_controller_test.php, the tearDownAfterClass() function should just be removed, as it's not doing anything excepting reassigning phpbb_root_path to this non-existent variable.