After migrating to ios 9 sandbox switch for Game Center has gone.
Before ios 9 there was developer (sandbox) and live environment and while production one had to create standalone sandbox user, switch sandbox option and test applications. Also it was dangerous to mix live and sandbox logging and accounts which may lead to misc problems.
What should I do now to test game center?
You should do nothing special.
It appears that sandbox switch has gone because live and sandbox environment were merged at ios 9. I have found this explanation at apple developer's forum (https://forums.developer.apple.com/thread/5611). Haven't checked presentation and didn't found any particular written document, but what I did was logging in game center with my live profile and it worked!
So development build + live profile (the one I use as regular player) gave me insight in live environment with live leaderboards. I suppose it's ok and now developers will have less pain integrating game center. Also I can't imagine any kind of possible conflicts for this new scheme.
