Hi Paula, congratulations on making it through to an interview! They must think you've got the right stuff to spend the time finding out more about your skills, so well done. =)
If they're including a test as part of the interview, it generally means they're looking for someone with fairly specific skills (rather than a typical UX all-rounder), or someone that fits into their particular team culture. As ASHM suggests, this isn't really something you can study for - you'll either be who they need, or not. =) Don't take it at all personally - you're lucky not to get the job if you're not the right fit. Nothing worse than being stuck in a job that isn't right for you! If they've mentioned specific skills in the job advertisement then my guess is they'll test for something related to those.
UX/UI is a pretty broad role description, and sadly often just a trendy way of asking for a UI designer. The fact that they're including a test with the interview is hopeful that they also take the UX part of the role seriously, which is a good thing!
UX interviews/tests might be an hour or so long, and cover anything from a heuristic review of the interviewer's own website/product through to a fictional design task (such as asking you to design an app for some sort of common task). There may be more than one interviewer (I've heard of six, but that's a lot). They'll often give you a one-page brief and also allow you to ask some questions before you get started doing some sketches, presenting them, and then iterating on them. They're looking for how you can innovate, how you communicate, explain and defend your decisions and opinions, as well as get some insights into how you problem solve. Of course it's not supposed to be a realistic project as it's difficult to get a solid idea of the context in one session and you probably won't have any ability to do user research or testing, but it can be a useful way for them to get an idea of how you work. They will take all of the unrealistic expectations into account - no-one is going to expect a full and perfect product as a result. It's more about seeing your process and familiarity with UX design - the qualities of you as a designer-person, not necessarily the artefacts that you generate.
My main advice would be: do whatever you need to do to feel confident and prepared for the interview. When you're there breathe slowly and deeply so you can be calm and think clearly. Don't get too stressed about it, but give it a good shot.
I would also:
* Research the company, their team and their history
* Bring in something that helps you tell your UX story - portfolio (hardcopy or on iPad), project artefacts, whatever.
* Have a pile of stories on hand about other UX-related projects you've worked on (especially ones that illustrate process and principles)
* Use the whiteboard during the interview, if there is one there (and if you've used them in the past and are confident using it during your test)
* Don't be afraid to ask questions. If you're doing ALL the talking, you're possibly not listening or asking enough questions to engage the interviewer(s).
* Be real and let yourself shine.
Very happy to answer any other questions you might have before Monday, so fire away!