[thci] find border agents with non link-local addresses (#7853)

Thread BR may advertise IPv6 GUA rather than LLA. For example, Avahi
stops publishing LLA when any GUA is found on the network interface.

This commit enhances mdns_query to find border agents with
non-link-local addresses.

tests/scripts/thread-cert/find_border_agents.py

@@ -40,7 +40,7 @@ def on_service_state_change(zeroconf, service_type, name, state_change):
 class BorderAgent(object):
     alias = None
     server_name = None
-    link_local_addr = None
+    addr = None
     port = None
     thread_status = None
 
@@ -48,19 +48,30 @@ class BorderAgent(object):
         self.alias = alias
 
     def __repr__(self):
-        return str([self.alias, self.link_local_addr, self.port, self.thread_status])
+        return str([self.alias, self.addr, self.port, self.thread_status])
+
+
+def get_ipaddr_priority(addr: ipaddress.IPv6Address):
+    # calculate the priority of IPv6 addresses in order: Global > non Global > Link local
+    if addr.is_link_local:
+        return 0
+
+    if not addr.is_global:
+        return 1
+
+    return 2
 
 
 def parse_cache(cache):
     border_agents = []
 
     # Find all border routers
-    for ptr in cache['_meshcop._udp.local.']:
+    for ptr in cache.get('_meshcop._udp.local.', []):
         border_agents.append(BorderAgent(ptr.alias))
 
     # Find server name, port and Thread Interface status for each border router
     for ba in border_agents:
-        for record in cache[ba.alias.lower()]:
+        for record in cache.get(ba.alias.lower(), []):
             if isinstance(record, DNSService):
                 ba.server_name = record.server
                 ba.port = record.port
@@ -69,14 +80,16 @@ def parse_cache(cache):
                 sb = text.split(b'sb=')[1][0:4]
                 ba.thread_status = (sb[3] & 0x18) >> 3
 
-    # Find link local address for each border router
+    # Find IPv6 address for each border router
     for ba in border_agents:
-        for record in cache[ba.server_name.lower()]:
+        for record in cache.get(ba.server_name.lower(), []):
             if isinstance(record, DNSAddress):
                 addr = ipaddress.ip_address(record.address)
-                if isinstance(addr, ipaddress.IPv6Address) and addr.is_link_local:
-                    ba.link_local_addr = str(addr)
-                    break
+                if not isinstance(addr, ipaddress.IPv6Address) or addr.is_multicast or addr.is_loopback:
+                    continue
+
+                if not ba.addr or get_ipaddr_priority(addr) > get_ipaddr_priority(ipaddress.IPv6Address(ba.addr)):
+                    ba.addr = str(addr)
 
     return border_agents