runtime: fix test on windows

The test prints an excessive \n when /dev/null is not present.

R=golang-codereviews, bradfitz, dave
CC=golang-codereviews
https://golang.org/cl/54890043

src/pkg/runtime/proc_test.go

@@ -268,7 +268,7 @@ func main() {
 	if os.IsNotExist(err) {
 		// This test tests what it is intended to test only if writes are fast.
 		// If there is no /dev/null, we just don't execute the test.
-		fmt.Println("OK\n")
+		fmt.Println("OK")
 		return
 	}
 	if err != nil {