[tests] `mcast6.py` to receive UDP messages without `JOIN_GROUP` (#7757)
[tests] `mcast6.py` to receive UDP messages without `JOIN_GROUP` (#7757)

`mcast6.py` is used by GitHub Actions CI to send or receive UDP
messages in multicast groups.

This commit adds a `-u` argument to allow `mcast6.py` to receive UDP
messages without joining a multicast group.
srickardti/openthread,srickardti/openthread,jwhui/openthread,abtink/openthread,openthread/openthread,abtink/openthread,srickardti/openthread,jwhui/openthread,openthread/openthread,jwhui/openthread,abtink/openthread,openthread/openthread,abtink/openthread,openthread/openthread,srickardti/openthread,jwhui/openthread