Codeigniter: how to test library that takes Controller instance in constructor?

I have the following Library called Item_service:

class Item_Service {
private $items;

function __construct(Items $items)
{
    $this->items = $items;
}

Where Items is an instance of controllers/Items.php. In my test class called Items_test.php i try to load the Item_service library using the following line:

$items = $this->CI->load->library('items/Item_service');

But then I get error saying

TypeError: Argument 1 passed to Item_Service::__construct() must be an instance of Items, null given

My test class is written based on kenjis/ci-phpunit-test.

How should I inject the controller into this library to be able to test the library? Also, can I test the controller itself? There are a lot of helper methods inside the controller.