tests: add plugin for simple invoice delay.

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

tests/plugins/hold_invoice.py

@@ -0,0 +1,18 @@
+#!/usr/bin/env python3
+"""Simple plugin to allow testing while closing of HTLC is delayed.
+"""
+
+from lightning import Plugin
+import time
+
+plugin = Plugin()
+
+
+@plugin.hook('invoice_payment')
+def on_payment(payment, plugin):
+    time.sleep(float(plugin.get_option('holdtime')))
+    return {}
+
+
+plugin.add_option('holdtime', '10', 'The time to hold invoice for.')
+plugin.run()