Not a knowledgeable opinion, BUT I thought test pressings were on a normal compound and very suitable for play; in fact, it would seem they would use super-duper, double virgin vinyl just so they could hear any flaws. Makes sense that they would offer superior sonics because the stamper is so new. Wonder if you could ever get a test pressing that was subsequently rejected???
Again, this is just vague recollection. I thought Absolute Sounds was selling test pressings for big bucks, but couldn't find the link.