First of all, I would challenge the assumption that TDD and game development don't mix. While there are some things in game development which are hard to unit-test (How do you write a proper unit test for a graphic effect where the only formal requirement is "looks cool"?) there are several other areas where it does make sense. Your core game rules, for example.
But even more useful than automated unit tests are automated integration tests. Create a framework for your game which allows you to automatically:
- Load predefined game scenes
- Simulate them for a few seconds, simulating player input if necessary (preferably with enhanced speed)
- Check if the outcome of the scenes is the outcome you expect and report if it is not
When your game uses a random number generator, make sure you seed it with the same value every test run.
Create such a test suite for every feature and create a way to run all your tests automatically. As an example, here is the test suit of the game Factorio in action:
Just imagine testing every single feature you see in that 84 seconds video manually. Sure, no automated test suit can replace real flesh-and-blood testplayers, but it can still save you hundreds of play hours and drastically speed up your change-test-fix cycle if you have a fully automatic test suit.