Ok, you've officially taken the propaganda phrases too far, because this one doesn't even make sense. The purpose of an algebra test is ability to do algebra? The purpose of an intelligence test is pattern recognition, working memory, and some other things? Huh?

The right thing to say would be "The purpose of a test is what it does", in keeping with the phrase you modified this from, but that's arguably false too. The purpose of a test is what it was designed to do, and what it does is what it actually does.

You could say that the purpose of our continued usage of a test is to encourage what it actually does, regardless of what it was designed to do; which is what I believe you're trying to say, and I would agree with that statement. But I object to the thoughtless propagation of dumb phrases like the one you used.