I'm not familiar with shaders and game engines, so I'm not sure what you are saying.
I had a quick look at godot tests, and seems to me they cover some parts of the shaders?
Anyway, I was more wondering who/how people are dogmatic about TDD, and manage to leave out one out of three core concepts from red/green/refactor ?
If I cannot write shader code without broken tests, there is a bunch of yak shaving to make testing possible in first place, and that only covers a small subset of graphics pipeline features.
It also takes zero consideration for the interactive nature of games/graphics development.