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()

tests/test_plugin.py

@@ -127,7 +127,9 @@ def test_plugin_hook(node_factory, executor):
     assert(end_time >= start_time + 20)
 
 
-def test_plugin_notifications(node_factory):
+def test_plugin_connect_notifications(node_factory):
+    """ test 'connect' and 'disconnect' notifications
+    """
     l1, l2 = node_factory.get_nodes(2, opts={'plugin': 'contrib/plugins/helloworld.py'})
 
     l1.connect(l2)
@@ -418,3 +420,34 @@ def test_htlc_accepted_hook_forward_restart(node_factory, executor):
     l2.restart()
 
     f1.result()
+
+
+def test_warning_notification(node_factory):
+    """ test 'warning' notifications
+    """
+    l1 = node_factory.get_node(options={'plugin': 'tests/plugins/pretend_badlog.py'})
+
+    # 1. test 'warn' level
+    event = "Test warning notification(for unusual event)"
+    l1.rpc.call('pretendbad', {'event': event, 'level': 'warn'})
+
+    # ensure an unusual log_entry was produced by 'pretendunusual' method
+    l1.daemon.wait_for_log('plugin-pretend_badlog.py Test warning notification\\(for unusual event\\)')
+
+    # now wait for notification
+    l1.daemon.wait_for_log('plugin-pretend_badlog.py Received warning')
+    l1.daemon.wait_for_log('plugin-pretend_badlog.py level: warn')
+    l1.daemon.wait_for_log('plugin-pretend_badlog.py time: *')
+    l1.daemon.wait_for_log('plugin-pretend_badlog.py source: plugin-pretend_badlog.py')
+    l1.daemon.wait_for_log('plugin-pretend_badlog.py log: Test warning notification\\(for unusual event\\)')
+
+    # 2. test 'error' level, steps like above
+    event = "Test warning notification(for broken event)"
+    l1.rpc.call('pretendbad', {'event': event, 'level': 'error'})
+    l1.daemon.wait_for_log('plugin-pretend_badlog.py Test warning notification\\(for broken event\\)')
+
+    l1.daemon.wait_for_log('plugin-pretend_badlog.py Received warning')
+    l1.daemon.wait_for_log('plugin-pretend_badlog.py level: error')
+    l1.daemon.wait_for_log('plugin-pretend_badlog.py time: *')
+    l1.daemon.wait_for_log('plugin-pretend_badlog.py source: plugin-pretend_badlog.py')
+    l1.daemon.wait_for_log('plugin-pretend_badlog.py log: Test warning notification\\(for broken event\\)')