Preallocate created slice in S3 tests

In case drvr.PutContent fails and returns error we'd have
some extra memory allocated, though in this case
(test with known size of the slice being iterated), that's fine.

Signed-off-by: Milos Gajdos <milosthegajdos@gmail.com>

registry/storage/driver/s3-aws/s3_test.go

@@ -500,8 +500,7 @@ func TestWalk(t *testing.T) {
 	}
 
 	// create file structure matching fileset above
-	// nolint:prealloc
-	var created []string
+	created := make([]string, 0, len(fileset))
 	for _, p := range fileset {
 		err := drvr.PutContent(context.Background(), p, []byte("content "+p))
 		if err != nil {