`displayNotification` is an internal method; that's not what we should test here.

The real bug that we fix is that `NotificationScreen.incExternalNotifications` was not called when `addToDownloadsQueue` was called and the notification was not there.

So we need to test two things now:
- the notification is shown when we call `addToDownloadsQueue` and the timeout in `addToUpdatesQueue` was not yet triggerred; test both that the notification is displayed and that `NotificationScreen.incExternalNotifications` was called
- the notification is not shown when we call `addToDownloadsQueue` and the timeout in `addToUpdatesQueue` was triggerred;  test that `NotificationScreen.incExternalNotifications` was not called (you can test that the notification is still displayed of course)