pytest: test for accepting even unknown messages.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

tests/plugins/allow_even_msgs.py

@@ -0,0 +1,13 @@
+#!/usr/bin/env python3
+from pyln.client import Plugin
+
+plugin = Plugin(custom_msgs=[43690])
+
+
+@plugin.hook('custommsg')
+def on_custommsg(peer_id, payload, plugin, message=None, **kwargs):
+    plugin.log("Got message {}".format(int(payload[:4], 16)))
+    return {'result': 'continue'}
+
+
+plugin.run()