Found this in the Grog Blog

[Sarah]

If you get a test invite, you get to start ALL over. It's more like a job. The purpose of test is to...test. It's not about leveling and mastering. Interesting to see if they send out another batch of invites, it is lonely there, and a lot of things are hard to check out by yourself.

[MacIronhawk]

This only talks about signing up for test. I never got an invite for test tonotown, I just signed up and got picked.
Since I was in beta for POTCO I just got it when the game released.

Sorry, but signing up doesn't mean you'll become a test member.

P.S. They can take away your test account. When I signed up for toontown test and made it I got an email saying they could drop your test account at anytime. You should have a better chance of keeping your test account if you reprot bugs on test and don't get banned on open.