As far as skills tests, I’m not familiar with these tests. If it’s a design test, I recently had a discussion with other designers. The consensus was that employers want to see a lot of ideas rather then any one single idea in detail. I know that might sound strange. Their attitude about working is what you need to key on.
My discussion was about tests to compare applicants for a job. If you’re part of a smaller test group, they might have a different agenda.