I'm in agreement with the comment that Dumbledore was NOT trying to test Harry's capabilities - I think you're forgetting that this is Dumbledore you're talking about. He knows a person, their quirks, the way their mind works, and what they have the potential to do. There would be no reason for him to lessen the strength of the protection to the stone just to see if Harry could do it. No reason.

I think, while they don't seem severe, the Devil's Snare, the key challenge, and the chess game were extremely dangerous to someone who didn't have a clue what they were doing. But even so, I think these served mainly as deterents: they were supposed to put off whoever was looking for the stone, to send them back the way they came from because they were so harsh. Voldemort knew he wanted the stone but, not knowing what task was going to be next, he didn't know whether advancing would be worth putting what was left of himself through so much danger and physical stress, especially while it is clear he COULD return without it. And like the others have said, the Mirror of Erised is obstacle enough - Voldemort would never, ever have been able to get the stone because he planned on using it.

Though, that brings me to a theory I had always pondered ... if the thing was that the mirror would reveal the stone to someone who wanted to have it but didn't want to use it, why couldn't one of Voldemort's followers get the stone for him? They certainly would want it (to give to him), but they would never use it on themselves, nor be the ones who would use it for Voldemort.