Someone who can comprehend and communicate their thoughts. I need feedback that isn’t just “I don’t know, it wasn’t fun.”. Anyone who can do that and test as needed works for me; I’ll pick out some people to test experiences, some to find bugs, and some to consider design changes.
Regarding the rest of your questions, I’d just put everything you’ve done. Include everything, the number of visits for what you tested isn’t entirely or even necessarily your fault. Your job is to test the experience, not make it successful.
I look for testers who can think outside the box on ways to shred my game(s) to pieces and be honest about their opinions on what I make. Testers are more than bug hunters; they’re the first people who get to play my game(s) from the perspective of normal players. If a potential tester does barely anything to actually test things and doesn’t give actually valuable feedback, then I may turn them away.