Unless there are according to many forums about this topic, there is just one correct way to set up your /application/config.php file:
$config['modules_locations'] = array(
APPPATH.'modules/' => '../modules/',
APPPATH.'modules/test/' => '../modules/test/',
);
After setting that, I was be able to access the controller baz from the example on domain.com/index.php/baz (not /test/baz, thats also possible by adding a route for it.)