Autoloading of controller class names is not something you get access to in your application or have much of a need for except in a case like this.
You will need to manually include/require the file that contains the controller you wish to extend.
<?php
require_once 'UserController.php'; // no adjustment to this path should be necessary
class ArticleController extends UserController
{
// ...
}
Note that your view scripts will still be served from views/scripts/article not views/scripts/user. You can adjust the view path in each action if necessary.
As stated in the comment, you shouldn't have to change the path of the require_once statement, but you can change it as necessary (e.g. require_once APPLICATION_PATH . '/modules/test/controllers/UserController.php';
)