pytest: Add a plugin for custommsgs and check that they get the msgs

This completes the custommsg epic, finally we are back where we began all that
time ago (about 4 hours really...): in a plugin that implements some custom
logic.

tests/plugins/custommsg.py

@@ -0,0 +1,15 @@
+#!/usr/bin/env python3
+from pyln.client import Plugin
+
+plugin = Plugin()
+
+
+@plugin.hook('custommsg')
+def on_custommsg(peer_id, message, plugin, **kwargs):
+    plugin.log("Got a custom message {msg} from peer {peer_id}".format(
+        msg=message,
+        peer_id=peer_id
+    ))
+
+
+plugin.run()

tests/test_misc.py

@@ -2072,8 +2072,10 @@ def test_sendcustommsg(node_factory):
     and we can't send to it.
 
     """
-    l1, l2, l3 = node_factory.line_graph(3, opts={'log-level': 'io'})
-    l4 = node_factory.get_node(options={'log-level': 'io'})
+    plugin = os.path.join(os.path.dirname(__file__), "plugins", "custommsg.py")
+    opts = {'log-level': 'io', 'plugin': plugin}
+    l1, l2, l3 = node_factory.line_graph(3, opts=opts)
+    l4 = node_factory.get_node(options=opts)
     l2.connect(l4)
     l3.stop()
     msg = r'ff' * 32
@@ -2122,3 +2124,6 @@ def test_sendcustommsg(node_factory):
         )
     )
     l4.daemon.wait_for_log(r'\[IN\] {}'.format(serialized))
+    l4.daemon.wait_for_log(
+        r'Got a custom message {serialized} from peer {peer_id}'.format(
+            serialized=serialized, peer_id=l2.info['id']))