Quick fix to make session work for a single client in tests

Client.php

<?php

namespace MyVendor\MyBundleBundle\Test;

use Symfony\Bundle\FrameworkBundle\Client as BaseClient;
use Symfony\Component\BrowserKit\Cookie;

class Client extends BaseClient
{
    /**
     * Makes a request.
     *
     * @param Request $request A Request instance
     *
     * @return Response A Response instance
     */
    protected function doRequest($request)
    {
        // create the session cookie for the client if it does not exist
        if (null === $this->cookieJar->get(session_name())) {
            $this->cookieJar->set(new Cookie(session_name(), uniqid()));
        }

        $response = parent::doRequest($request);

        // save the session after performing the request
        $request->getSession()->save();

        return $response;
    }
}