There's a comment on HN from a Rare employee going into how they do their testing, for example - https://news.ycombinator.com/item?id=14802509
Edit: It's a 5 year old thread and I have a few comments in there... Fuck I'm old.
In my experience, unit testing is _incredibly_ rare, and Rare are pretty much the only studio that I'm aware of that do it.