Stop timeout thread in test.py when main thread exits

Change-Id: I7354f4ed96abd55ce2d661a57e6548309f6b43f3
diff --git a/tools/test.py b/tools/test.py
index bd5916d..c59a23a 100755
--- a/tools/test.py
+++ b/tools/test.py
@@ -382,7 +382,8 @@
     if utils.is_python3():
       threading.Thread(
           target=timeout_handler,
-          args=(timestamp_file, print_stacks_timeout)).start()
+          args=(timestamp_file, print_stacks_timeout),
+          daemon=True).start()
     else:
       thread.start_new_thread(
           timeout_handler, (timestamp_file, print_stacks_timeout,))