pytest: Add a test for the commitment_revocation hook

tests/plugins/watchtower.py

@@ -0,0 +1,14 @@
+#!/usr/bin/env python3
+
+from pyln.client import Plugin
+
+plugin = Plugin()
+
+
+@plugin.hook('commitment_revocation')
+def on_commitment_revocation(commitment_txid, penalty_tx, plugin, **kwargs):
+    with open('watchtower.csv', 'a') as f:
+        f.write("{}, {}\n".format(commitment_txid, penalty_tx))
+
+
+plugin.run()