Remove test.go to make SDK importable

pkg/zxchainsdk/certificate_flow.go

@@ -372,7 +372,7 @@ func UploadCertificatePNGToCOS(ctx context.Context, pngBytes []byte, defaultObje
 	_, err = client.Object.Put(ctx, objectKey, bytes.NewReader(pngBytes), &cos.ObjectPutOptions{
 		ObjectPutHeaderOptions: &cos.ObjectPutHeaderOptions{
 			ContentType:   "image/png",
-			ContentLength: len(pngBytes),
+			ContentLength: int64(len(pngBytes)),
 			CacheControl:  "public, max-age=31536000, immutable",
 		},
 	})