Er, this is Pedants' Corner, somebloke! I disagree with your 'ultimate test'. Most of my inlaws would say, "Have you drawed a nice picture?", or "I haven't broke it.". Just because someone says something in a particular way all the time, it doesn't mean that they are correct!
Suzie - yes I agree there, but I do think that language is not all logical. For example the phrase "by and large" or the statement "It's early days yet" are both widely accepted correct idioms - and not just regionalisms or slang, and yet when you try to analyse them they don't make sense.
I tend to draw a parallel with music. There are rules of harmony and the default position is to obey them, but the tutored ear is the final judge.
I certainly wouldn't interpret this as an "anything goes" attitude. I do think knowledge of the rules is important, even if you end up breaking them on occasion.
I quite like "Fowler's Modern English Usage" on this issue. His discussion of the split infinitive is particularly good I think.
Have you read about the guy who set the test? It explains a lot. He's essentially a Latinist who has set himself up as an authority on English grammar, and is much beloved of Michael Gove and Prince Charles.