btest/core/suspend_processing: Add WebSocket example

Add a test/example forwarding all new_connection() events produced during
`zeek -r wikipedia.trace` as my_new_connection() to a WebSocket client.

This is mostly to demonstrate and verify usage of suspend_processing(),
websocket_client_added(), resume_processing(), Pcap::file_done(),
websocket_client_lost() and terminate() together.

testing/btest/Files/ws/wstest.py

@@ -33,8 +33,14 @@ import time
 from typing import Any, Callable, Optional, Union
 
 import websockets.sync.client
+from websockets.exceptions import (
+    ConnectionClosedError,
+    ConnectionClosedOK,
+)
 from websockets.sync.client import ClientConnection
 
+_ = ConnectionClosedOK, ConnectionClosedError
+
 WS_PORT = (
     int(os.environ["WEBSOCKET_PORT"].split("/")[0])
     if "WEBSOCKET_PORT" in os.environ