TestImportExtremelyLargeImageWorks: optimize DevZero

According to https://github.com/golang/go/issues/5373, go recognizes
(and optimizes for) the following syntax:

```go
for i := range b {
	b[i] = 0
}
```

so let's use it. Limited testing shows ~7.5x speed increase,
compared to the previously used syntax.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

internal/testutil/helpers.go

@@ -20,8 +20,8 @@ var DevZero io.Reader = devZero{}
 type devZero struct{}
 
 func (d devZero) Read(p []byte) (n int, err error) {
-	for i := 0; i < len(p); i++ {
-		p[i] = '\x00'
+	for i := range p {
+		p[i] = 0
 	}
 	return len(p), nil
 }