It's a diagnostic test. Read the actual words.
It's not an exam, it's a tool they use to determine what if any additional maths support you might need - you don't need to (and indeed, probably shouldn't) prepare for it, as that will enable them to figure out what support you'll need for the coming term/year/degree and ensure you do learn the necessary material.
Most likely if you "fail" (which you can't because it's not going to be summative assessment) they will provide additional materials on their VLE for you to review and/or may arrange additional tutorials if there are a group of people with weaker background, to catch everyone up.