Is it hard ? Sure.
But in my personal experience, most programmers err heavily on the side of failing to test thoroughly something that will become a long-term solution, not on the side of over-testing their throwaways.
You can write a good set of tests when you know the initial requirements, and not all requirements iterate at the same rate as the code. As you learn new requirements, you write new tests. The old ones don't go away (nor do the early requirements if they're good), even if you completely replace the implementing code.
Throwaway prototypes are part of the requirement discovery process.