tests: rtpsession: refactor tests to use GstHarness
tests: rtpsession: refactor tests to use GstHarness

This patch simplifies the tests (44% less code) and
makes them much more readable.

The provided SessionHarness also makes it much easier
to write new tests for rtpsession.

https://bugzilla.gnome.org/show_bug.cgi?id=791070

StreamUtils/gst-plugins-good,GStreamer/gst-plugins-good,GStreamer/gst-plugins-good,GrokImageCompression/gst-plugins-good,pexip/gst-plugins-good,StreamUtils/gst-plugins-good,GStreamer/gst-plugins-good,GrokImageCompression/gst-plugins-good,pexip/gst-plugins-good,StreamUtils/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,StreamUtils/gst-plugins-good,GStreamer/gst-plugins-good,GrokImageCompression/gst-plugins-good,GrokImageCompression/gst-plugins-good,pexip/gst-plugins-good