I guess they changed it in iOS 7. You used to logout from the actual gameCenter app but I don’t see that, so I guess it has to be done from settings.
As far as the second part. I don’t think there is anything special about iTunes IAP test accounts other than they don’t get charged when you try to buy things. But underneath the hood they are just other AppleID
See: http://stackoverflow.com/questions/8331005/how-do-i-test-game-center