What's the difference between adjectives "supplementary" and "supplemental"?
I think you've answered your own question! The additional test questions are "added to supply what is wanted" in this case more of them. So it should be "This is a page of Supplementary questions"
Supplemental is used more in the sense of adding value or function or information. A further definition of a word, more functionality to a process.
Supplementary is adding more of the same without necessarily improving the whole.