Add an integration test for VNC + websockify

[yuvipanda]

This tests the following components:

1. The VNC server
2. websockify
3. Jupyter Server Proxy
4. The desktop environment itself

It does so by:

1. Building the container image
2. Starting a container with that image, with jupyter server running inside.
3. Connecting to the container via websockets, authenticating and exposing the websocket connection as a regular TCP socket on localhost via websocat
4. Taking a screenshot of the remote desktop by connecting to it through vncdotool. This requires a tcp connection, hence the double proxying with websocat
5. Verify that the image produced by the screenshot is the same as a reference image we have.

This only misses the noVNC + JS from testing, but otherwise represents a massive improvement to the status quo!

[github-actions[bot]]

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[yuvipanda]

This works perfectly fine locally, but unfortunately something is causing it to hang when the VNC client is trying to connect to the server on GitHub actions. will have to debug next time i get a chance.

[yuvipanda]

Not needed, now that we have #119