pytest: Add a test for the 'warning' subscription and notification

1. Create a plugin: ./lightning/tests/plugins/pretend_badlog.py
This plugin subscribes 'warning' notification and log the payload of
'warning';
2. Add a new test: tests/test_plugin.py::test_warning_notification
This test runs the plugin-pretend_badlog.py and check if 'warning'
notification can be normal triggered and subscribed.

tests/plugins/pretend_badlog.py

@@ -0,0 +1,32 @@
+#!/usr/bin/env python3
+"""This plugin is used to check that warning(unusual/broken level log) calls are working correctly.
+"""
+from lightning import Plugin
+
+plugin = Plugin()
+
+
+@plugin.init()
+def init(configuration, options, plugin):
+    plugin.log("initialized")
+
+
+@plugin.subscribe("warning")
+def notify_warning(plugin, warning):
+    plugin.log("Received warning")
+    plugin.log("level: {}".format(warning['level']))
+    plugin.log("time: {}".format(warning['time']))
+    plugin.log("source: {}".format(warning['source']))
+    plugin.log("log: {}".format(warning['log']))
+
+
+@plugin.method("pretendbad")
+def pretend_bad(event, level, plugin):
+    """Log an specified level entry.
+    And in plugin, we use 'warn'/'error' instead of
+    'unusual'/'broken'
+    """
+    plugin.log("{}".format(event), level)
+
+
+plugin.run()