harness: add _set_forwarding function

To be able to disable the slightly "magic" forwarding of the
necessary events between the harnesses.

Also introduce a new test-suite for GstHarness, that documents the
feature, and should hopefully expand into documenting most of the
features the harness possesses.

https://bugzilla.gnome.org/show_bug.cgi?id=752746
6 files changed