Use `httpx` as client on WebSocket tests about uvicorn

[kaypee90]

@Kludex is the idea to use this kind of implementation https://gist.github.com/tomchristie/3293d5b118b5646ce79cc074976744b0 for the WebSocket tests

[Kludex]

Yes. Some tweak is needed to use app=app.

[kaypee90]

@Kludex app is passed to the config like this

config = Config(
        app=app,
        ws=ws_protocol_cls,
        http=http_protocol_cls,
        lifespan="off",
        port=unused_tcp_port,
    )

Which is then passed to run_server async with run_server(config):
No sure why you need app=app for this

[likecodingloveproblems]

I have opened a PR for this issue