Testing with sessions in symfony

Accoring to documentation I'm trying to mock session in my PHPUnit test:

public function testAction()
{
    $session = new Session(new MockArraySessionStorage());
    $session->set('abc', 'xyz');
    $client = static::createClient();
    $client->getContainer()->set('session', $session);
    $client->request('GET', '/');
}

And in my controller I trying to get session value:

public function mainAction()
{
    $session = $this->get('session');
    var_dump($session->get('abc'), $session->all());die; //returns null and []
}

Why this stuff doesn't work?