Session attributes returns null in controller while testing in Symfony2

I'm testing my controller functions with Phpunit in Symfony 2.8. I set some attributes in my session using MockFileSessionStorage() and it works fine during my first request, but then when I send a form I can't reach my attributes in my controller, it returns null.

Here's my test :

class CheckoutControllerTest extends WebTestCase
{
    public function setClient()
    {
        return $client = static::createClient(array(), array(
            'PHP_AUTH_USER' => static::DEMO_USER,
            'PHP_AUTH_PW' => static::DEMO_PASSWORD,
        ));
    }


    public function testSetupCart()
    {
        $client = $this->setClient();

        //set session
        $session = new Session(new MockFileSessionStorage());
        $container = $client->getContainer();

        $session->set("shopbundle_cart_id", "1");
        $session->set("shopbundle_current_event_hash", "hash");
        $container->set('session', $session);

        $crawler = $client->request('GET', 'http://ift.tt/2dZuFLB');

        $this->assertTrue($client->getResponse()->isSuccessful());
        //I can get my session attributes in my controller

        $form = $crawler->selectButton('Continue')->form();

        $client->submit($form);
        //in my controller handling the form submit, i can't get my attributes

        $client->followRedirect();
        $this->assertTrue($client->getResponse()->isSuccessful());

    }

I access the attributes in both controllers like this : $cartId = $this->get("session")->get("shopbundle_cart_id", null);. The form is submitted in POST.

Here is my config_test.yml file :

imports:
    - { resource: config_dev.yml }

framework:
    test: ~
    session:
        storage_id: session.storage.mock_file
    profiler:
        collect: false

web_profiler:
    toolbar: false
    intercept_redirects: false

swiftmailer:
    disable_delivery: true

Can you help me finding out what is wrong ?