It depends what the job is. I'm guessing they'll just want to see how fluent you are with the software. I suspect there'll be tasks along the lines of "Create this and make it look like this" to see if you understand the different ways of producing results. To be honest though I'd expect them to be testing your creativity and thinking rather than just Photoshop software skills, but I guess it depends on the role you're going for.
If you're going in as an apprentice they cant expect you to be overly proficient with the software (that's why you're an apprentice and not a junior) so I'd guess it'll be more to check that you can handle the basics and how you then think about the task in hand.
At the end of the day don't stress too much about it, go in, do what you can do and have a stab at the stuff you're not sure about. You'll probably find you do a lot better if you have a little confidence in yourself and stay calm.
hey guys! so the test, there was 4 of us sat in a room, they gave us a brief which was to create a wedding invitation for a hand towel, they gave you specifications , information and so on, from then we had an hour to complete, once we had complete the brief we were then given a sheet of paper with some questions on, we then had to get into pairs and ask each other these questions, learn abit more about each other, once we had done that we had to explain our work and that was the end of the interview, no 1 to 1, but they did say that they will be inviting at least 2 people back for a second interview, so fingers crossed, i felt it went well.