[release-branch.go1.4] go/build: build $GOOS_test.go always

««« CL 176290043 / 8025b7d1e6c9
go/build: build $GOOS_test.go always

We decided to build $GOOS.go always
but forgot to test $GOOS_test.go.

Fixes issue 9159.

LGTM=r
R=r
CC=golang-codereviews
https://codereview.appspot.com/176290043
»»»

LGTM=r
R=r
CC=golang-codereviews
https://codereview.appspot.com/182740043