cmd/go: skip TestBuildOutputToDevNull on Plan 9

TestBuildOutputToDevNull was added in CL 16585.
However, copying to /dev/null couldn't work on Plan 9,
because /dev/null is a regular file. Since it's not
different from any other file, the logic in copyFile
couldn't distinguish it from another, already existing,
file, that we wouldn't want to overwrite.

Change-Id: Ie8d353f318fedfc7cfb9541fed00a2397e232592
Reviewed-on: https://go-review.googlesource.com/16691Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: David du Colombier <0intro@gmail.com>

src/cmd/go/go_test.go

@@ -1167,6 +1167,9 @@ func TestInstallIntoGOPATH(t *testing.T) {
 
 // Issue 12407
 func TestBuildOutputToDevNull(t *testing.T) {
+	if runtime.GOOS == "plan9" {
+		t.Skip("skipping because /dev/null is a regular file on plan9")
+	}
 	tg := testgo(t)
 	defer tg.cleanup()
 	tg.setenv("GOPATH", filepath.Join(tg.pwd(), "testdata"))