testutil, storage: use math/rand.Read where possible

Use the much faster math/rand.Read function where cryptographic
guarantees are not required. The unit test suite should speed up a
little bit but we've already optimized around this, so it may not
matter.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

registry/storage/filereader_test.go

@@ -2,7 +2,6 @@ package storage
 
 import (
 	"bytes"
-	"crypto/rand"
 	"io"
 	mrand "math/rand"
 	"os"
@@ -16,7 +15,7 @@ import (
 func TestSimpleRead(t *testing.T) {
 	ctx := context.Background()
 	content := make([]byte, 1<<20)
-	n, err := rand.Read(content)
+	n, err := mrand.Read(content)
 	if err != nil {
 		t.Fatalf("unexpected error building random data: %v", err)
 	}