test: avoid output from "go test" when tests pass

Some tests where printing to os.Stdout as a side effect even if the
output was not used/checked in the test.

app_test.go

@@ -5,6 +5,7 @@ import (
 	"flag"
 	"fmt"
 	"io"
+	"io/ioutil"
 	"os"
 	"strings"
 	"testing"
@@ -556,6 +557,7 @@ func TestAppNoHelpFlag(t *testing.T) {
 	HelpFlag = BoolFlag{}
 
 	app := NewApp()
+	app.Writer = ioutil.Discard
 	err := app.Run([]string{"test", "-h"})
 
 	if err != flag.ErrHelp {