yii2/tests/framework/web/ErrorHandlerTest.php

<?php

namespace yiiunit\framework\web;

use Yii;
use yii\web\NotFoundHttpException;
use yiiunit\TestCase;

class ErrorHandlerTest extends TestCase
{
    protected function setUp()
    {
        parent::setUp();
        $this->mockWebApplication([
            'components' => [
                'errorHandler' => [
                    'class' => 'yiiunit\framework\web\ErrorHandler',
                    'errorView' => '@yiiunit/data/views/errorHandler.php',
                ],
            ],
        ]);
    }

    public function testCorrectResponseCodeInErrorView()
       {
           /** @var ErrorHandler $handler */
           $handler = Yii::$app->getErrorHandler();
           $this->invokeMethod($handler, 'renderException', [new NotFoundHttpException('This message is displayed to end user')]);
           $out = Yii::$app->response->data;
           $this->assertEquals('Code: 404
Message: This message is displayed to end user
Exception: yii\web\NotFoundHttpException', $out);
       }
}

class ErrorHandler extends \yii\web\ErrorHandler
{
    /**
     * @return bool if simple HTML should be rendered
     */
    protected function shouldRenderSimpleHtml()
    {
        return false;
    }
}