Add debug logs for outgoing DNS messages

Since AsyncDnsServer logs incoming DNS messages as seen on the wire, do
the same for the responses sent by the server.

(cherry picked from commit 2a9c74546d)

bin/tests/system/isctest/asyncserver.py

@@ -580,6 +580,7 @@ class AsyncDnsServer(AsyncServer):
         peer = Peer(addr[0], addr[1])
         responses = self._handle_query(wire, peer, DnsProtocol.UDP)
         async for response in responses:
+            logging.debug("Sending UDP message: %s", response.hex())
             transport.sendto(response, addr)
 
     async def _handle_tcp(
@@ -672,6 +673,7 @@ class AsyncDnsServer(AsyncServer):
     ) -> None:
         responses = self._handle_query(wire, peer, DnsProtocol.TCP)
         async for response in responses:
+            logging.debug("Sending TCP response: %s", response.hex())
             writer.write(response)
             await writer.drain()