Guide testing

The start of a testing guide. This PR relies on the crates and modules one because I shuffled some stuff around, so sorry about that.

I got stuck with how to import this name with cargo. @wycats and @alexcrichton , any ideas?
Filed as https://github.com/rust-lang/rust/issues/16078, there's other places where we need to do this, and I'd like to tackle that all at once.