>>Maybe if your code is "Hello World", but what if your code is a motion search, or a facial recognition algorithm, or a band-pass filter, or any other sort of real-world code whose operation can't be summed up exactly in two lines of code?

Then you break the problem down as much as possible so that you know exactly what should be the expected output for the expected input. You have to keep breaking the problem down until it becomes obvious how to automate the tests. Otherwise it'll be really hard to write good, clean, modular code.