[ticket/11405] Add some basic tests for notifications in submit_post()

Poster, should NOT receive a notification
Topic subscribed, should receive a notification
Topic subscribed, but unauthed to read, should NOT receive a notification
Topic subscribed, but already notified, should NOT receive a new notification
Topic and forum subscribed, should receive ONE notification
Forum subscribed, should receive a notification
Forum subscribed, but already notified, should NOT receive a new notification

PHPBB3-11405
[ticket/11405] Send post notifications to forum subscribers

Like in 3.0 we should also send notifications about new posts to users
that subscribed to the forum. (Subscriptions are verbose)

PHPBB3-11405