Ciris: To the best of my knowledge, no.
I'm not sure if this isn't part of why CD PROJEKT RED made sure to announce the system requirements for The Witcher 3 loud enough - so that everyone can make sure if they have good enough systems to run it. And yes, it's not the same as actually testing a game out, but...
To be honest, I don't know when I last heard about a demo for a game being created. It seems like the rather distant past :D but I may be wrong on that one.
Well, when was the last time a company released a AAA grade game without mandatory online clients or other DRM? Well... ? That's right. A thing of the distant past. :D
CDPR like to handle things a little differently, and above the current industry standard, after all.
Anyhow, a full demo isn't necessary, but I'd also like to have some sort of benchmark test program, just so we can see how the game and its various engine features would perform on our systems. Yes, they've released requirements, but it's no secret that those are usually a vague guideline at best. As far as I know they also didn't clearly specify just what kind of performance we can expect with the required hardware specs. ( Resolution, FPS etc. ) A practical test would be the most reliable and customer friendly option, especially considering the rather high requirements, and the fact that many people will have to upgrade their system just to run the game reasonably well.