Deallocate GObject* with g_object_unref instead of gst_object_unref.
Even if it works now, it is confusing and in the future it might
not work if any GstObject specifics are added.
https://bugzilla.gnome.org/show_bug.cgi?id=762142
Adds unit tests similar to the ones that we have for DASH and HLS.
Tests:
* manifest parsing finishes successfully
* some queries (duration, seekable, latency)
* seeking with various values and flags