testing: structure package documentation
testing: structure package documentation

Makes finding the relevant information much easier.

R=golang-dev, adg
CC=golang-dev
https://codereview.appspot.com/8353045

Committer: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang