Symfony2 functional test and session persistance

Here is my problem, I add session variable from functional test controller and they don't popup on action targeted by test.

I made a login method and used cookbook advice to log in. I tuned it a bit to allow 2 accounts login : admin and superadmin

/**
     * @param $account string the account to log in with (authorized : superadmin and admin)
     */
    protected function logIn($account)
    {
        $this->session = $this->container->get('session');
        // Cookbook try
        // $this->session = new Session(new MockFileSessionStorage());
        $user = $this->em->getRepository('LCHUserBundle:User')->findOneByUsername($account);
        $firewall = 'admin_area';

        switch($account) {
            case self::SUPER_ADMIN_LOGIN:
                $token = new UsernamePasswordToken($user, $account, $firewall, array('ROLE_SUPER_ADMIN'));
                $this->client->setServerParameter("SERVER_NAME", SiteControllerTest::ROOT_SITE);
                $this->client->setServerParameter("HTTP_HOST", SiteControllerTest::ROOT_SITE);
                break;
            case self::ADMIN_LOGIN:
                $token = new UsernamePasswordToken($user, $account, $firewall, array('ROLE_ADMIN'));

                // Session var I wish to have on my controller action tested
                $this->session->set('currentSite', $this->em->getRepository('LCHMultisiteBundle:Site')->find(1));
                $this->session->save();

                // Use to force server canonical name for admin test
                $this->client->setServerParameter("SERVER_NAME", SiteControllerTest::ROOT_SITE);
                $this->client->setServerParameter("HTTP_HOST", SiteControllerTest::TEST_SITE);
                break;
            default:
                throw new UsernameNotFoundException('Username provided doesn\'t match any authorized account');
        }
        // Save user in session
        $this->session->set('_security_'.$firewall, serialize($token));
        $this->session->set('user', $user);
        $this->session->save();
        // $this->container->set('session', $this->session);

        $cookie = new Cookie($this->session->getName(), $this->session->getId());
        $this->client->getCookieJar()->set($cookie);

My setUp() method does this :

/**
     * {@inheritDoc}
     */
    protected function setUp()
    {
        // Initiates client
        $this->client = static::createClient();

        $this->container = $this->client->getContainer();
        $this->application = new Application($this->client->getKernel());
        $this->application->setAutoExit(false);
        $this->translator = $this->container->get('translator');
        $this->em = $this->container
            ->get('doctrine')
            ->getManager();
    }

You can see that I set session vars for authentication. They appear correctly when I dump session var from tested action, but if I add my currentSite session var, it seems not persisted. As I use the test container provided by client, it should be passed on shouldn't it?

PS : I also overrided Client class, according to this other question.

I found numerous posts on topic but none provide any working solution (this one, this one or that one). I also found this cookbook article.