Add values to the session array

I'm having a controller which reads an integer from the session array in this manner:

if ($session->has('administration')) { $id = $session->get('entity_id'); return $this->getDoctrine()->getRepository('AppBundle:SomeEntity')->find($id); }

Currently I'm writing tests for this controller action but I can't find how to inject this integer in the sessions array. I tried the following:

$this->client = $this->makeClient(true); $this->client->getContainer()->set('session.storage.options.entity_id', 12);

I also tried the following:
$this->client = $this->makeClient(true); $session = new Session(new MockFileSessionStorage()); $session->set('administration', 12); $this->client->getContainer()->set('session', $session);

This, however doesn't work and seems to just ignore the value. Therefore, my question is how to store values in the session array in a functional test environment?

Start with the functional test setup using makeClient(true), the client container, and Symfony's Session with MockFileSessionStorage. Determine how a session value can be made available to the controller during the test, then verify that the controller reads entity_id and resolves the entity. Done means the session value is reliably available in the functional test.