[5.8] Add assert method for unauthorized response

[ramytamer]

This PR adds a new assertion method on test response like assertNotFound and assertForbidden; this will help asserting that an endpoint is limited only to authorized users instead of doing it as assertStatus(401).

There is no isUnauthorized method defined in Symfony/http-foundation Response class like isForbidden, isNotFound yet so that's why I compare it with the 401 HTTP code directly.