[scripts] test multicast routing across Thread and Backbone (#5852)

- This commit fixes some issue in multicast forwarding from Thread to
  Backbone.
- It also adds a test to verify that multicast forwarding works across
  Thread and Backbone.
  - Test script tests/scripts/thread-cert/mcast6.py is added for Host
    to subscribe to a multicast address

The ping reply can not reach ROUTER1 since DUA feature is not
complete.

tests/scripts/thread-cert/pktverify/layer_fields.py

@@ -385,6 +385,7 @@ _LAYER_FIELDS = {
     'ipv6.opt.router_alert': _auto,
     'ipv6.opt.padn': _str,
     'ipv6.opt.length': _list(_auto),
+    'ipv6.opt.mpl.seed_id': _bytes,
     'ipv6.opt.mpl.sequence': _auto,
     'ipv6.opt.mpl.flag.v': _auto,
     'ipv6.opt.mpl.flag.s': _auto,