Test output port notification iterator

Test output port notification iterator

Added tests are:

* You can create an output port notification iterator and the
  notifications it consumes and returns are the expected ones.

* You cannot run a graph once you have created a notification iterator
  on the output port of one of its components.

* You can create an output port notification iterator, subscribe to
  specific types of notifications, and the notifications it consumes and
  returns are the expected ones.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>