The problem with games and unit testing is that games are ultimately about being fun, and 'being fun' is not a well-specified domain.
Here's a story. You are on a team implementing a calculator game. You're in charge of 'Addition', and you create an extensive test suite covering all sort of properties of addition over natural numbers. Commutativity. Associativity. Identity. You've even handled all possible floating point weirdness. Good job.
The next day, the game designer meets with you. You’ll have to add in integers, bringing in subtraction and negative numbers. No sweat. You go back, fix the tests that guard against all that negativity. All tests are green.
The next day, the game designer meets with you again. "We took a look over our analytics, and nobody likes using a 5. Our beta testers hate it and moan about it on our Discord. People rage quit when they have to type in 55. We need to change our addition engine so that all 5s are removed."
"Huh?"
"Yeah, just get rid of 'em. All 5s, everywhere. Don't wanna see them anymore."
"What's the answer be to 2 + 3 then?"
"Hmm, good question. Let's make it 6."
You're a bit perplexed as to why, but you go back and dutifully change the code and then all the tests. 5 are now illegal everywhere, and you implement addition in a base-9 system that successfully avoids using the forbidden digit.