Honestly I've never seen this problem solved well for any game engine short of in-house engines(which obviously wouldn't break titles they were supporting). It

Honestly I've never seen this problem solved well for any game engine short of in-house engines(which obviously wouldn't break titles they were supporting). It's been a while since I've been in the industry so maybe things have changed but last time I was there unit tests weren't even a thing. Smoke tests where if a set of levels loaded and didn't crash was considered success from an automation validation standpoint.