test: try to implement websocket server 409 integration test

[filmaj]

⚠️ WIP!

This PR attempts to write a test encapsulating the issue reported in https://github.com/slackapi/bolt-python/issues/1203, in an attempt to better understand how bolt behaves in the situation where the websocket server backend returns a 409 error on initial handshake. I think I am not doing this correctly, though.. probably need a closer look from the Python experts.