Would this be much use even if it existed?
Wouldn't you be better off just buying another card - a mid-range card is cheap as . And then you get to use your os, your own dev tools, and are able test your whole application properly.
About all such a sandbox would do is let you try some trivial opencl coding without a graphics card - which one can already do with one of the cpu backends.
You can try www.hgpu.org. AMD has a high-end system available here for experimenters. You can try running some applications here to get some insight about the speed-ups you might get. But as notzd said, best is to buy a medium level card.