test: rtpbin_buffer_list: check if the chain_list function has been called
test: rtpbin_buffer_list: check if the chain_list function has been called

Make the test more useful to verify that the chain list function has
actually been called.

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