image/jpeg: use strings.Builder

Change-Id: I0a51aa9e7800689c123ce3eaf74742a4641b7681
Reviewed-on: https://go-review.googlesource.com/c/go/+/428261
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/image/jpeg/dct_test.go

@@ -5,10 +5,10 @@
 package jpeg
 
 import (
-	"bytes"
 	"fmt"
 	"math"
 	"math/rand"
+	"strings"
 	"testing"
 )
 
@@ -181,7 +181,7 @@ func slowIDCT(b *block) {
 }
 
 func (b *block) String() string {
-	s := bytes.NewBuffer(nil)
+	s := &strings.Builder{}
 	fmt.Fprintf(s, "{\n")
 	for y := 0; y < 8; y++ {
 		fmt.Fprintf(s, "\t")

src/image/jpeg/reader_test.go

@@ -177,7 +177,7 @@ func check(bounds image.Rectangle, pix0, pix1 []byte, stride0, stride1 int) erro
 }
 
 func pixString(pix []byte, stride, x, y int) string {
-	s := bytes.NewBuffer(nil)
+	s := &strings.Builder{}
 	for j := 0; j < 8; j++ {
 		fmt.Fprintf(s, "\t")
 		for i := 0; i < 8; i++ {

src/image/jpeg/writer_test.go

@@ -13,6 +13,7 @@ import (
 	"io"
 	"math/rand"
 	"os"
+	"strings"
 	"testing"
 )
 
@@ -82,7 +83,7 @@ func TestUnscaledQuant(t *testing.T) {
 	}
 	if bad {
 		names := [nQuantIndex]string{"Luminance", "Chrominance"}
-		buf := &bytes.Buffer{}
+		buf := &strings.Builder{}
 		for i, name := range names {
 			fmt.Fprintf(buf, "// %s.\n{\n", name)
 			for zig := 0; zig < blockSize; zig++ {