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?
The sections about attributes and stability attributes are fairly long. Perhaps they should be moved out of the testing section to avoid breaking the flow. (I propose to place them later and add a reference)