Harness.pushFromSrc

Transfer data from the src-#GstHarness to the main-#GstHarness. It consists of 4 steps: 1: Make sure the src is started. (see: gst_harness_play) 2: Crank the clock (see: gst_harness_crank_single_clock_wait) 3: Pull a #GstBuffer from the src-#GstHarness (see: gst_harness_pull) 4: Push the same #GstBuffer into the main-#GstHarness (see: gst_harness_push)

MT safe.