tests: Update mp4 mux test for mdat placeholder change
tests: Update mp4 mux test for mdat placeholder change

The mp4 muxer now writes a place-holder mdat as a free
atom followed by a 0-byte mdat that covers the rest of the
file, making it possible to rewrite it as 64-bit, or leave
it as-is if nothing else is written afterward

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