public function testForm()
{
    $client = self::createClient();
    $client->loginUser(new InMemoryUser('admin', null));

    $session = $client->getSession();
    $session->set('_csrf/form', '123456789');
    $session->set('foo', 'bar');
    $session->save();

    $client->request('POST', '/form', ['form' => ['_token' => '123456789']]);
}

array:3 [
  "_security_main" => "O:52:"Symfony\Bundle\FrameworkBundle\Test\TestBrowserToken":2:{i:0;s:4:"main";i:1;a:5:{i:0;O:49:"Symfony\Component\Security\Core\User\InMemoryUser":4:{s:59:"\x00Symfony\Component\Security\Core\User\InMemoryUser\x00username";s:5:"admin";s:59:"\x00Symfony\Component\Security\Core\User\InMemoryUser\x00password";N;s:56:"\x00Symfony\Component\Security\Core\User\InMemoryUser\x00roles";a:0:{}s:58:"\x00Symfony\Component\Security\Core\User\InMemoryUser\x00enabled";b:1;}i:1;b:1;i:2;N;i:3;a:0:{}i:4;a:0:{}}}",
  "_csrf/form" => "123456789",
  "foo" => "bar"
]