test: Fix intermittent issue in p2p_handshake.py

If we reuse the same port when disconnecting and establishing connections
again, we might hit this scenario:
- disconnection is done on python side for P2PConnection
- disconnection is not complete on c++ side for TestNode
- we're trying to establish a new connection on same port again

Prevent this scenario from happening by ensuring disconnection on c++
side for TestNode as well.

Author: stratospher

test/functional/p2p_handshake.py

@@ -41,6 +41,7 @@ def add_outbound_connection(self, node, connection_type, services, wait_for_disc
             peer.sync_with_ping()
             peer.peer_disconnect()
             peer.wait_for_disconnect()
+        self.wait_until(lambda: len(node.getpeerinfo()) == 0)
 
     def test_desirable_service_flags(self, node, service_flag_tests, desirable_service_flags, expect_disconnect):
         """Check that connecting to a peer either fails or succeeds depending on its offered