pytest: test the reply functionality (via blinded path) using a plugin.

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

tests/plugins/onionmessage-reply.py

@@ -0,0 +1,21 @@
+#!/usr/bin/env python3
+"""
+This plugin is used to test the `onion_message` hook.
+"""
+from lightning import Plugin
+
+plugin = Plugin()
+
+
+@plugin.hook("onion_message")
+def on_onion_message(plugin, onion_message, **kwargs):
+    if 'reply_path' not in onion_message:
+        plugin.log("no reply path")
+        return
+
+    plugin.rpc.call('sendonionmessage', [onion_message['reply_path']])
+    plugin.log("Sent reply via {}".format(onion_message['reply_path']))
+    return {"result": "continue"}
+
+
+plugin.run()