[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/bytes.py

@@ -27,13 +27,13 @@
 #  POSSIBILITY OF SUCH DAMAGE.
 #
 import sys
-from typing import Union
+from typing import Union, Any
 
 
 class Bytes(bytearray):
     """Bytes represents a byte array which is able to handle strings of flexible formats"""
 
-    def __init__(self, s: Union[str, bytearray, 'Bytes']):
+    def __init__(self, s: Union[str, bytearray, 'Bytes', Any]):
         if isinstance(s, str):
             try:
                 s = Bytes._parse_compact(s)