cmd/go, testing: revise docs for test flags

In cmd/go's 'go help testflag':
* Rewrite list of flags to drop test. prefix on every name.
* Sort list of flags.
* Add example of using -bench to match all benchmarks.

In testing:
* Remove mention of undefined 'CPU group' concept.

Fixes issue 4488.
Fixes issue 4508.

R=adg
CC=golang-dev
https://codereview.appspot.com/7288053