Well the time I met Tim (seniuor producer) he DID say it is difficult to test the game because it's such an "open" game and also they need to test the ep on it's own, how it will function with the core game and the previous ep's autnomously as wellas all together...
(I'm not sure I'm explaining this right ) either ways bottom line is thath the game is an open game and thath there are too many possiblities to test them all. He also admited that new things in new EP's may make it possible thath other things in older ep's won't function as they should and they try their best to make it all work...