If you want testable code, the first step is to separate computations from effects. Most of your program should be immutable. Ideally you'd have a mostly functional core, used by an imperative shell.

Now to test a function, you just give it inputs, and check the outputs. Simple as that.

Oh you're worried that your function might use some other function, and you still want to test it in isolation? I said give it up. Instead, test that other function first, and when you're confident it's bug free (at least for the relevant use cases), then test your first function.

And in the rare cases where you still need to inject dependencies, remember that most languages support some form of currying.