I have never been formally "tested" for any belt, nor have I ever seen it done that way at any school. What I mean is, knowing that you are being tested for promotion. All of my promotions, belts and stripes, have been somewhat of a surprise to me, excluding 1 stripe.
I was awarded my blue belt one night after a long training session where I trained with many senior belts and then was able to effectively use that on other students of my grade.
For purple, our class was playing a "game" where the class is lined up by size, not rank. The two smallest guys would fight 2 min, winner fights next in line. I was 2nd from biggest of about a dozen guys, and somehow fought my way up and down the line almost twice. After about a 5 min stalemate match with another purple belt, I was awarded my purple.
I think "testing" for a purple belt is not how it should be. Yes, there are concepts and techniques you should know or be aware of at each level, but being exposed to these techniques is hardly grounds for promotion. I think effective application in a real scenario, as well ability to defend against are equally as important.
Also the ability to teach it to another student in a simple and comprehensible manner, and the ability for them to understand and apply is really important the higher you get, especially if you plan to teach. As you become more advanced in your school, and the field of equal belts shrinks, lower belts will look up to you and ask you questions. You must be able to answer them.