Usage of all commands is printed to stderr, except go test, which is printed to
stdout. This is inconsistent.

Print `go test -help` to stderr instead.

R=rsc@golang.org

Change-Id: I079f4788134bf9aedcccc26838879eedad1c925e
Reviewed-on: https://go-review.googlesource.com/15434Reviewed-by: Andrew Gerrand <adg@golang.org>