pytest: add test for a plugin which falls over outside a command.

This actually passes fine, but it's an interesting case to test.

Fixed-by: Darosior <darosior@protonmail.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

tests/plugins/fail_by_itself.py

@@ -0,0 +1,31 @@
+#!/usr/bin/env python3
+from pyln.client import Plugin
+import os
+import threading
+import time
+
+plugin = Plugin()
+
+
+class FailThread(threading.Thread):
+    def __init__(self):
+        super().__init__()
+        self.start()
+
+    def run(self):
+        time.sleep(1)
+        print("Exiting!")
+        os._exit(1)
+
+
+@plugin.init()
+def init(options, configuration, plugin):
+    FailThread()
+
+
+@plugin.method('failcmd')
+def failcmd(plugin):
+    pass
+
+
+plugin.run()