Originally 'go test -h' printed the output of 'go help test'.
Then issue #6576 was filed, because that output didn't list (for example) -bench.
CL 14502065 changed 'go test -h' to print the output of 'go help testflag'.
Then issue #9209 was filed, because that output didn't list (for example) -c.

To print all the relevant flags, parts of both 'go help test' and 'go help testflag'
are needed. Refactor the help messages to make those parts available
and print them.

Fixes #9209.

Change-Id: Ie8205b8fb37d00c10d25b3fc98f14286ec46c4e3
Reviewed-on: https://go-review.googlesource.com/12173Reviewed-by: Rob Pike <r@golang.org>