test: Fix intermittent failure in rpc_net.py

The wait in disconnect_p2ps checked for subver, which
is unavailable for the peer that didn't send a version msg.

test/functional/test_framework/test_node.py

@@ -656,7 +656,8 @@ class TestNode():
         return len([peer for peer in self.getpeerinfo() if peer['subver'] == P2P_SUBVERSION])
 
     def disconnect_p2ps(self):
-        """Close all p2p connections to the node."""
+        """Close all p2p connections to the node.
+        Use only after each p2p has sent a version message to ensure the wait works."""
         for p in self.p2ps:
             p.peer_disconnect()
         del self.p2ps[:]