Imagine you make some code that needs to comply to a law, but then the law changes. Or client decides he wants his notification in different format, look shape.
And you've spent 2x the effort that you would've had you not written the test in the first place.
My initial comment was meant to be a statement about the relative chance that you're solving the wrong problem vs. solving the problem wrong. Prototypes help you identify that you're solving the wrong problem, unit tests help you identify if you're solving the problem wrong. In the beginning of a project, you are much, much more likely to be solving the wrong problem than solving the problem wrong. Go ensure that the system works end-to-end and solves the user's needs before you make it bulletproof.
