I'll start with of course this was an April fool's joke. Wow.
But for those getting excited at the prospect...
Of course, writing tests takes about 100th of the time of designing application structure and the writing code, doesn't it? The productivity gains would be nothing like as high as the suggestion.
My guess would be that best one could hope for in a tool such as this (when it finally does appear) would be around doubling the output.
Having something write code for you may be fantasy today, but reality soon; the productivity gains are just PURE fantasy.