The build tags are necessary to keep "go build" in that directory
building only stdio.go, but we have to arrange for test/run.go to
treat them as satisfied.

Fixes #12625.

Change-Id: Iec0cb2fdc2c9b24a4e0530be25e940aa0cc9552e
Reviewed-on: https://go-review.googlesource.com/17454
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>