tests: test that we only use tlv onion when advertized.

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

tests/plugins/print_htlc_onion.py

@@ -0,0 +1,19 @@
+#!/usr/bin/env python3
+"""Plugin that prints out HTLC onions.
+
+We use this to check whether they're TLV or not
+
+"""
+
+from lightning import Plugin
+
+plugin = Plugin()
+
+
+@plugin.hook("htlc_accepted")
+def on_htlc_accepted(htlc, onion, plugin, **kwargs):
+    plugin.log("Got onion {}".format(onion))
+    return {'result': 'continue'}
+
+
+plugin.run()