On the page listing available tests (http://www.javablackbelt.com/TestDefListing.wwa), mousing over the black bar next to an exam says "You haven't took this exam yet."

This is not grammatically correct English. It should be "You haven't taken this exam yet." While "took" makes sense logically, we have many exceptions. So "You took the exam" is correct English, but adding the word "have" changes the rules.