GstDiscoverer objects were leaked by tests making the leaks detector
unusable.
Introduce ges_deinit(), similiar to gst_deinit(), doing some cleanup
before exiting the process.
https://bugzilla.gnome.org/show_bug.cgi?id=776805
And fix all the tests as we need to wait for the project to be loaded
to check the reference count of the timeline (as we keep a ref on the
timeline in project to later emit "loaded" on idle).