Laravel测试：协调http请求和控制器

Question

在下面的测试中，我们（非常谦卑）需要确保$utmSource类字段收到正确的值。

<?php

class ExampleTest extends TestCase {

  public function testHttpRequestAndControllerCoordination() {

    $response = $this->call('GET', '/')->json([utm_source => 'Google']);
    $topPageController = new TopPageController();
    $topPageController->renderTopPage();

    $this->assertAttributeSame('Google', 'utm_source', $topPageController);
  }
}

class TopPageController extends Controller {

    private $utmSource;

    public function renderTopPage(){
        $this->utmSource = request()->utm_source;
    }
}

当然，该测试不会通过，因为$response和$topPageController是独立的，所以$utmSource将是null。我们如何关联$response和$topPageController？