Bingo! I did serialize data structures and put them in my tests. In my case it was Python so everything was serialized into a readable textual format. Maybe use snappy compressed json? Being purely functional and having a fairly flat data structures really helped, I didn't have to serialize very far down. Another side effect of being flat is the garbage collector has less work to do. Reallocate early and often!
Amen. I've wanted this so many times in Java. A really hacky way to do this is to serialize your objects while debugging and deserialize them in your test. The problem is if the classes change your test won't work because you can't deserialize anymore. Encapsulation and data hiding really get in the way here.