Functional testing as authenticated user

protected static function createAuthenticatedClient($username, $options = [], $server = [])
{
    $client = static::createClient($options, $server);
    $container = $client->getContainer();

    $session = $container->get('session');
    /** @var $userManager \FOS\UserBundle\Doctrine\UserManager */
    $userManager = $container->get('fos_user.user_manager');
    /** @var $loginManager \FOS\UserBundle\Security\LoginManager */
    $loginManager = $container->get('fos_user.security.login_manager');
    $firewallName = $container->getParameter('fos_user.firewall_name');

    $user = $userManager->findUserBy(array('username' => $username));
    $loginManager->loginUser($firewallName, $user);

    // save the login token into the session and put it in a cookie
    $container->get('session')->set(
        '_security_' . $firewallName,
        serialize($container->get('security.context')->getToken())
    );
    $container->get('session')->save();
    $client->getCookieJar()->set(new Cookie($session->getName(), $session->getId()));

    return $client;
}

Source: https://advancingusability.wordpress.com/2013/11/15/functional-testing-with-authentication-and-symfony-2-3-fosuserbundle/