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 #4488.
Fixes #4508.

R=adg
CC=golang-dev
https://golang.org/cl/7288053