Testing Mailer with mock

gistfile1.php

class Mailer
{
    /**
     * @var \Swift_Mailer
     */
    private $mailer;
    /**
     * @var \Twig_Environment
     */
    private $twig;
    /**
     * @var string
     */
    private $mailFrom;

    public function __construct(
        \Swift_Mailer $mailer,
        \Twig_Environment $twig,
        string $mailFrom)
    {
        $this->mailer = $mailer;
        $this->twig = $twig;
        $this->mailFrom = $mailFrom;
    }

    public function sendConfirmationEmail(User $user)
    {
        $body = $this->twig->render(
            'email/registration.html.twig', [
            'user' => $user
        ]);

        $message  = (new \Swift_Message())
            ->setSubject('Welcome to the micro-post app')
            ->setFrom($this->mailFrom)
            ->setTo($user->getEmail())
            ->setBody($body, 'text/html');

        $this->mailer->send($message);
    }
}

class MailerTest extends TestCase
{
    public function testConfirmationEmail()
    {
        $user = new User();
        $user->setEmail('miro@miro.com');

        $swiftMailer = $this->getMockBuilder(\Swift_Mailer::class)
            ->disableOriginalConstructor()
            ->getMock();

        $swiftMailer->expects($this->once())->method('send')
            ->with($this->callback(function ($subject) {
                $messageStr = (string)$subject;

                return strpos($messageStr, "From: me@domain.com") !== false
                    && strpos($messageStr, 'Content-Type: text/html; charset=utf-8') !== false
                    && strpos($messageStr, 'Subject: Welcome to the micro-post app') !==false
                    && strpos($messageStr, 'To: miro@miro.com') !==false
                    && strpos($messageStr, 'This is a message body') !==false;
            }));

        $twigMock = $this->getMockBuilder(\Twig_Environment::class)
            ->disableOriginalConstructor()
            ->getMock();
        $twigMock->expects($this->once())->method('render')
            ->with('email/registration.html.twig',
                [
                    'user' => $user
                ]
            )->willReturn('This is a message body');

        $mailer = new Mailer($swiftMailer, $twigMock, 'me@domain.com');
        $mailer->sendConfirmationEmail($user);
    }
}