Laravel Unit Testing Dependency Injection

I figured it out. Here is the updated class.

class ShoppingCartTest extends TestCase {

    use DatabaseTransactions;

    protected $shoppingCart;

    public function setUp() {

        parent::setUp();

        $this->shoppingCart = $this->app->make('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);
    }

}

Thanks to @edcs for guiding me in the right direction. You need to use a setUp function and not __construct as the app instance hasn't been created yet.