StupidSandbox doesn't really work

[prandla]

StupidSandbox doesn't have any tests, and it's missing some APIs that evaluation_step uses, so it doesn't work currently.

We should either fix it and add tests for it, or remove it entirely. I guess it might be easier for testing as you don't need to set up isolate, but I'm not sure how many people test like that (you can't test anything related to sandboxes like that anyways).