tests: Fix rtprtx test by handling buffer lists
tests: Fix rtprtx test by handling buffer lists

Commit #1018aa made rtprtxsend handle buffer lists, breaking
the test which probes for buffers, but not buffer lists.

Use a utility function to run the probe callback on each buffer
in the list in turn and remove any buffers that are dropped.

rawoul/gst-plugins-good,rawoul/gst-plugins-good,pexip/gst-plugins-good,GrokImageCompression/gst-plugins-good,ikonst/gst-plugins-good,ikonst/gst-plugins-good,froggatt/gst-plugins-good-m,StreamUtils/gst-plugins-good,kittee/gst-plugins-good,hizukiayaka/gst-plugins-good,rawoul/gst-plugins-good,shelsonjava/gst-plugins-good,froggatt/gst-plugins-good-m,chamois94/gst-plugins-good,shelsonjava/gst-plugins-good,freedesktop-unofficial-mirror/gstreamer__gst-plugins-good,stfl/gst-plugins-good,GStreamer/gst-plugins-good,chamois94/gst-plugins-good,StreamUtils/gst-plugins-good,sebras/gst-plugins-good,shelsonjava/gst-plugins-good,chamois94/gst-plugins-good,stfl/gst-plugins-good,stfl/gst-plugins-good,pexip/gst-plugins-good,StreamUtils/gst-plugins-good,GrokImageCompression/gst-plugins-good,GStreamer/gst-plugins-good,vatavuserban/gst-plugins-good,kittee/gst-plugins-good,GrokImageCompression/gst-plugins-good,GrokImageCompression/gst-plugins-good,ikonst/gst-plugins-good,Kurento/gst-plugins-good,hizukiayaka/gst-plugins-good,freedesktop-unofficial-mirror/gstreamer__gst-plugins-good,sebras/gst-plugins-good,pexip/gst-plugins-good,GStreamer/gst-plugins-good,chamois94/gst-plugins-good,vatavuserban/gst-plugins-good,freedesktop-unofficial-mirror/gstreamer__gst-plugins-good,pexip/gst-plugins-good,Kurento/gst-plugins-good,vatavuserban/gst-plugins-good,Kurento/gst-plugins-good,sebras/gst-plugins-good,kittee/gst-plugins-good,froggatt/gst-plugins-good-m,stfl/gst-plugins-good,Kurento/gst-plugins-good,hizukiayaka/gst-plugins-good,GStreamer/gst-plugins-good,freedesktop-unofficial-mirror/gstreamer__gst-plugins-good,froggatt/gst-plugins-good-m,Kurento/gst-plugins-good,sebras/gst-plugins-good,ikonst/gst-plugins-good,shelsonjava/gst-plugins-good,hizukiayaka/gst-plugins-good,StreamUtils/gst-plugins-good,pexip/gst-plugins-good,rawoul/gst-plugins-good,vatavuserban/gst-plugins-good,kittee/gst-plugins-good