message: test: use NODE_* constants instead of magic numbers
  We just assigned `NODE_NETWORK | NODE_WITNESS` to `nServices` a few
  lines above. Use that for verifying correctness instead of `9`.
diff: @@ -36,7 +36,7 @@ class AddrReceiver(P2PInterface):
  def on_addrv2(self, message):
  for addr in message.addrs:
  - assert_equal(addr.nServices, 9)
  + assert_equal(addr.nServices, NODE_NETWORK | NODE_WITNESS)
  assert addr.ip.startswith('123.123.123.')
  assert (8333 <= addr.port < 8343)
  self.addrv2_received_and_checked = True
  