go: help messages for 'go test'

go: help messages for 'go test'

The plan is to make 'go test' replace gotest entirely, so it
cannot refer to gotest's godoc.  Instead, copy gotest's
documentation in as three different help messages:
'go help test', 'go help testflag', and 'go help testfunc'.

R=r
CC=golang-dev
https://golang.org/cl/5491048