cmd/go: guarantee all test output is on stdout

cmd/go: guarantee all test output is on stdout

In past releases, whether test output appears on stdout or stderr
has varied depending on exactly how go test was invoked and
also (indefensibly) on the number of CPUs available.
Standardize on standard output for all test output.
This is easy to explain and makes go test | go tool test2json work nicely.

Change-Id: I605641213fbc6c7ff49e1fd38a0f732045a8383d
Reviewed-on: https://go-review.googlesource.com/76871
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>