Prefer const/let over var

tests/test.localization.js

@@ -1,12 +1,11 @@
-var expect = chai.expect;
-
+const expect = chai.expect;
 import { l10n } from '../app/localization.js';
 
 describe('Localization', function() {
     "use strict";
 
     describe('language selection', function () {
-        var origNavigator;
+        let origNavigator;
         beforeEach(function () {
             // window.navigator is a protected read-only property in many
             // environments, so we need to redefine it whilst running these