Test.class.php

<?php

class Test
{
    public function __construct(){
        //$file = __FILE__;
        $file = dirname(__FILE__).'/test.php';
        register_uninstall_hook($file, array('Test', 'uninstall'));
    }

    public static function uninstall(){
        ob_start();
        var_dump(defined(WP_UNINSTALL_PLUGIN), WP_UNINSTALL_PLUGIN);
        throw new Exception(ob_get_clean());
    }
}