tests: allow ret_nodes to be global for mocha

src/tests/frontend/index.html

@@ -22,7 +22,7 @@
     <script src="lib/underscore.js"></script>
 
     <script src="lib/mocha.js"></script>
-    <script> mocha.setup({ui: 'bdd', checkLeaks: true, timeout: 60000}) </script>
+    <script> mocha.setup({ui: 'bdd', checkLeaks: true, globals: 'ret_nodes', timeout: 60000}) </script>
     <script src="lib/expect.js"></script>
 
     <script src="helper.js"></script>