`cargo test --no-default-features --features=rand-std` fails

[apoelstra]

It looks like the tests in #492 just aren't annotated properly, so my local tests fail. Not sure why I didn't notice this at the time I merged that PR.

cargo build succeeds (with some warnings about unreachable code) so I don't think this is a problem for any downstream crates.

[tcharding]

We can close this one, fixed by #520