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`.

test/functional/p2p_addrv2_relay.py

@@ -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