It was clear to me as well that the test was for Twilight. After all, her "magic makes it all complete". It was her binding of the elements wielded by her friends that enabled their true power. I believe it will be her helping them realize their true natures again that will lead to Disqord's defeat.
I'm hoping that Disqord doesn't get banished to where he cannot return or turned to stone. That's more Celestia's style, not Twilight's. Twilight would want somepony to learn and grow from their mistakes.
I will faint deadaway if the final lines of part 2 turn out to be:
TWILIGHT: Why do you use ponies for recreation?
DISQORD: If so, you've not provided the best.
TWILIGHT: Leave us! We've passed your little test.
DISQORD: Temper, temper, mon petit poney.
TWILIGHT: Get out of Equestria!
DISQORD: I do so only because it suits me to. But I will not promise never to appear again.
