pyln: add support for dependent hooks.

And use that to add simple tests.

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

tests/plugins/dep_c.py

@@ -0,0 +1,15 @@
+#!/usr/bin/env python3
+from pyln.client import Plugin
+
+"""A simple plugin that must come before dep_a.
+"""
+plugin = Plugin()
+
+
+@plugin.hook('htlc_accepted', before=['dep_a.py'])
+def on_htlc_accepted(htlc, plugin, **kwargs):
+    print("htlc_accepted called")
+    return {'result': 'continue'}
+
+
+plugin.run()