> I could write a test for my first pass at a function... but then if I decide I need to actually split that function into two or go for a different approach entirely, then I have to basically scrap that test.

Then you're writing a function coupled to the test, not writing logic you can plug into the test to reasonably verify it works as intended.

That's the bit people miss I think. Tests should reflect how the code is consumed, and test for that (inputs and outputs, more or less) not for implementation details. Your test shouldn't care if its 1 function or 7.