gst-validate: Don't treat FALSE when sending flushes as an error

gst_element_send_event(FLUSH_START / FLUSH_STOP) returns FALSE in cases where any of the most downstream elements have unlinked pads, even if the pipeline is successfully flushed.

Currently this is considered expected behavior in GStreamer. This patch updates gst-validate to treat it as such and therefore not fail the test for a "failing" flush.