harness: always set our test-clock on the harnessed element
harness: always set our test-clock on the harnessed element

The integration is already so tight, there is no reason to
not further formalize it!

https://bugzilla.gnome.org/show_bug.cgi?id=761914

centricular/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,centricular/gstreamer