helper/stub: copy args to stderr

Some helpers are implemented via go test itself in tests, and as a result stdout gets clobbered.

Signed-off-by: Ophestra <cat@gensokyo.uk>

helper/stub.go

@@ -74,7 +74,7 @@ func flagRestoreFiles(offset int, ap, sp string) (argsFile, statFile *os.File) {
 func genericStub(argsFile, statFile *os.File) {
 	if argsFile != nil {
 		// this output is checked by parent
-		if _, err := io.Copy(os.Stdout, argsFile); err != nil {
+		if _, err := io.Copy(os.Stderr, argsFile); err != nil {
 			panic("cannot read args: " + err.Error())
 		}
 	}