Maybe a new game mode is added where new or unreleased items are added (before going to private maps, creative, and survival multiplayer) Players can find glitches and problems with those blocks/Items. Not all features can make it into Testing because they might be features for specific modes, or used in main menu, stuff like that.
Single player mode used to server this purpose. However since single player is such a laggy, glitchy, and critically unplayable mode I don't think that anyone would willingly want to play on that mode for more than a few minutes (except for that one guy from a few months ago of course, lol).
BlueTangs Rock3,
Single player mode used to *serve this purpose.
Then add in like private maps or something