Fixed incorrect behavior DefaultToFIFOPriorityTest (Issue 6504)

DefaultToFIFOPriorityTest fills the queue with MAX requests at loop: 

    for (MockedRequestHandler h : requests) {
      final MockedRequestHandler req = h;
      new Thread(new Runnable() {  // Thread safety reviewed
        public void run() {
          req.process();
        }
      }).start();
    }

and expects that  the order in the queue (NewSessionRequestQueue newSessionQueue in Registry class) will match to order of the list (List<MockedRequestHandler> requests). But this isn't always true, because the order running threads is not defined. 

The test expects that first element in the queue will be same that in the list:
    while (cpt < 8) {
      try {
        requests.get(0).getSession();
        break;
      } catch (Throwable e) {
        // ignore.
      }
      Thread.sleep(250);
    }

if this not true, we get an infinite loop.

I propose to switch next request when previous already added to queue.