You could always include it using __DIR__:
include(dirname(__DIR__).'/config.php');
__DIR__ is a 'magical constant' and returns the directory of the current file without the trailing slash. It's actually an absolute path, you just have to concatenate the file name to __DIR__. In this case, as we need to ascend a directory we use PHP's dirname which ascends the file tree, and from here we can access config.php.
You could set the root path in this method too:
define('ROOT_PATH', dirname(__DIR__) . '/');
in test.php would set your root to be at the /root/ level.
include(ROOT_PATH.'config.php');
Should then work to include the config file from where you want.