In my view, product/code prototypes don't require testing as they are often delivered using a completely different process and for completely different purposes than a finished product.

Imagine this common scenario. You have some sort of technology that your company is famous for, your sales team or a chief exec goes to a client and is able to sell them a different product that is leveraging your technology. They sell it there and then and promise the client they'll see a beta version in a month time. You don't have anything, even a development team.

What happens next is you quickly design a solution based on the points agreed/sold to the client, assemble a team of (often star) engineers and start writing code. It would be ideal of course if your dates didn't slip (they always do) and you managed to write code in three weeks time rather than four. In this case you have an extra week, and if you were able to secure a QA team's time for this week - you will test it. In reality you won't have an extra week and/or your QA team is booked for another two months supporting other product deliveries. Moreover you won't have any design specs, so QA team won't have any idea what they are supposed to be testing. So you end up delivering what you wrote with a bit of your own smoke testing, which isn't really a testing as such, simply because you aren't qualified to test your own code. The biggest factor here, is that the client will see some of the promised functionality very quickly. If your prototype does the job, then you're back to the drawing board, where you design and plan the actual version 1.0 - properly this time.

As far as unit-tests go, they are good to maintain certain contracts (I promise my code will do this, if it doesn't - it's broken). In case of prototyping, you may not care so much for contracts, because often enough your prototypes will not live to become version 1.0, whether because the client has went away, or because you ended up developing your version 1.0 using different technologies/tools. An example here would be a quick prototype website written very quickly on PHP in a month's time by one person, and then you assemble a team to rewrite it in JEE. Even the database will often not survive this change.