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?
One idea is to  introduce Traits and Operators before you introduce Testing. Then you can write about `#[deriving(Eq)] `  as metioned in #16003.

I don't know if this is consistent with the master plan.