How to politely refuse note sharing
I think Mack put you in a very difficult situation. You should not have agreed to his terms, and should ask to change them now.

There is no problem sharing your notes with Mack, though he should probably be getting notes from several students.

You should either have agreed not to share your notes with others, or agreed to keep confidential the fact that they will be used in setting the test, but not both. If you were freely sharing your notes, nobody would pay any special attention to them. If you could say "Mack asked me not to share them so that he can use them in setting his test" people would understand the non-sharing.

There are other ways Mack could have constructed his test. For example, he could have asked each lecturer for a question or two with marking criteria.