[test] Fix WebsockifyServerHarness on ipv6 enabled machine. NFC

The test_nodejs_sockets_echo_subprotocol_runtime test was failing on
my machine because node was resolving "localhost" to the IPv6 address
::1 but the websockify server was running on the IPv4 address
127.0.0.1.

Author: sbc100

test/test_sockets.py

@@ -65,7 +65,7 @@ def __enter__(self):
 
     # start the websocket proxy
     print('running websockify on %d, forward to tcp %d' % (self.listen_port, self.target_port), file=sys.stderr)
-    wsp = websockify.WebSocketProxy(verbose=True, listen_port=self.listen_port, target_host="127.0.0.1", target_port=self.target_port, run_once=True)
+    wsp = websockify.WebSocketProxy(verbose=True, source_is_ipv6=True, listen_port=self.listen_port, target_host="127.0.0.1", target_port=self.target_port, run_once=True)
     self.websockify = multiprocessing.Process(target=wsp.start_server)
     self.websockify.start()
     self.processes.append(self.websockify)