One thing to consider here though is that often times when you realize splitting up a function will make it easier to test, it's because your implementation sucks and is doing too much and too tightly coupled. I mean realistically how would splitting up a function make testing it easier unless the function is already complex and performing multiple tasks?

You can certainly argue that some of the clean code folks do a lot of needless abstraction that makes it harder to work on code, and I think that's true at times. But at the same time, a 200 line method doing 19 different things is also quite hard to understand and modify, and the reason testers want to split that method up is because it's really hard to understand and has too many possible outcomes.

I don't like to overly abstract things and I try to strike a balance here, but I can say without a doubt that I've never found it harder to understand and work on a single class with 20 methods that each do one thing (with descriptive method names) than I have a method with 200 lines of code doing the same 20 things. And the former is much easier to test as well.