Has any one experience or recommendations for automating the testing of a game e.g using Selenium.

I suppose making such a machine/program wouldn't be too hard, just extremely time consuming. By the time you would have already completed your game twice over!

I have never made one myself, but I have heard of people making buttin pressers and AI before. Some have even made AI that can survive for a short time in Minecraft. But they have too much time on their hands already.

Not tried Selenium. I do some unit testing before an update. This is how I did it:
http://docs.textadventures.co.uk/quest/unit_testing.html