[test] Add testing for outbound feeler connections

Extend the addconnection RPC method to allow creating outbound
feeler connections. Extend the test framework to accept those
feeler connections.

test/functional/p2p_add_connections.py

@@ -14,6 +14,13 @@ def check_node_connections(*, node, num_in, num_out):
     assert_equal(info["connections_in"], num_in)
     assert_equal(info["connections_out"], num_out)
 
+class P2PFeelerReceiver(P2PInterface):
+    def on_version(self, message):
+        # The bitcoind node closes feeler connections as soon as a version
+        # message is received from the test framework. Don't send any responses
+        # to the node's version message since the connection will already be
+        # closed.
+        pass
 
 class P2PAddConnections(BitcoinTestFramework):
     def set_test_params(self):
@@ -91,6 +98,14 @@ class P2PAddConnections(BitcoinTestFramework):
 
         check_node_connections(node=self.nodes[1], num_in=5, num_out=10)
 
+        self.log.info("Add 1 feeler connection to node 0")
+        feeler_conn = self.nodes[0].add_outbound_p2p_connection(P2PFeelerReceiver(), p2p_idx=6, connection_type="feeler")
+
+        # Feeler connection is closed
+        assert not feeler_conn.is_connected
+
+        # Verify version message received
+        assert_equal(feeler_conn.message_count["version"], 1)
 
 if __name__ == '__main__':
     P2PAddConnections().main()