Laravel 单元测试依赖注入

I'm trying to write a test class for a shopping cart. Here is what I have:

ShoppingCartTest.php

class ShoppingCartTest extends TestCase {

    use DatabaseTransactions;

    protected $shoppingCart;

    public function __construct() {
        $this->shoppingCart = resolve('App\Classes\Billing\ShoppingCart');
    }

    /** @test */
    public function a_product_can_be_added_to_and_retrieved_from_the_shopping_cart() {

        // just a placeholder at the moment
        $this->assertTrue(true);
    }

}

但是，当我运行phpunit时，Laravel似乎无法解析我的ShoppingCartClass.

以下是错误:

Fatal error: Uncaught exception 'Illuminate\Contracts\Container\BindingResolutionException'
with message 'Unresolvable dependency resolving
[Parameter #0 [ <required> $app ]] in class Illuminate\Support\Manager'
in C:\Development Server\EasyPHP-Devserver-16.1\eds-www\nrponline\vendor\laravel\framework\src\Illuminate\Container\Container.php:850

我的ShoppingCart类在多个不同的控制器中得到了很好的解决.

Why can't Laravel resolve it during my tests?个

I refered to this post as well but still didn't have any luck.