rtpmanagerbad: rtpsrc do not set iface on sink
requested to merge den_erpel/gstreamer:gst-plugins-bad-bugfix/rtpmanagerbad-rtpsrc-mcast-iface into main
Copied from https://gitlab.freedesktop.org//gstreamer/gst-plugins-bad/-/merge_requests/1579
DynUdpSink does not have multicast-iface
Adjust test to verify this.