test: add EnsureFileSizeStream test

src/backupformat/tgz.js

@@ -1,11 +1,5 @@
 'use strict';
 
-exports = module.exports = {
-    getBackupFilePath,
-    download,
-    upload
-};
-
 const assert = require('assert'),
     BoxError = require('../boxerror.js'),
     DataLayout = require('../datalayout.js'),
@@ -272,3 +266,12 @@ async function upload(backupConfig, remotePath, dataLayout, progressCallback) {
         await tarPack(dataLayout, backupConfig.encryption, uploader, progressCallback);
     });
 }
+
+exports = module.exports = {
+    getBackupFilePath,
+    download,
+    upload,
+
+    // exported for testing
+    _EnsureFileSizeStream: EnsureFileSizeStream
+};