Or it could be that they have one more test regardless of whether he was pure of heart or not which is the dragon. And unless they do manage to find a new home for SG-1 we may never know!

I think this is moot. When we talk about the ancients and any technology based tests which these obviously were we cant possibly expect them to make a contraption that detects "evil" or "purity" these are mental constructs and attatchments of sentient beings not empiracal measurable data sets. In all of these tests they were designed so only those with such qualities would make the right decision leading to the passing of the test, or at least to widdle out some of the obvious undesireables, they didnt explicitly sense or test whether or not one possessed these qualities.