Don't worry, just be yourself and have all the correct info on your boat. If you don't axshully know the answer, you can show the book with the info in...... and demonstrate you can read.
Good Luck, dont forget to take the mugshot photographs with you
When I test members of our club for ICC, it takes about 2-3h on the water and covers all the points in the syllabus. I've not failed anyone, though I have once asked someone to take a week to review basic colregs and have a re-test on that before signing the form. Just keep cool and think "safety and listen to crew". Hope all goes well on the day!