Thanks for mentioning UserTest.io in your post, however, you have missed the mark with what UserTest.io is/does.
UserTest.io is a remote user testing platform that allows you to use ‘Your Own Testers’ or ‘Our Qualified Testers’. If you choose to use ‘Your Own Testers’, then of course you can pre-screen before hand. Likewise, with our ‘Qualified Website Reviewers’, you can define a set of pre-screener questions to select your target audience/users.
I’d recommend signing up and using the service :D
Best of luck!