Downgrade QUnit for Backbone test support.

package.json

@@ -45,7 +45,7 @@
     "markdown-doctest": "^0.8.0",
     "platform": "^1.3.1",
     "qunit-extras": "^2.0.1",
-    "qunitjs": "^2.0.0",
+    "qunitjs": "^1.23.1",
     "request": "^2.69.0",
     "requirejs": "^2.2.0",
     "sauce-tunnel": "^2.5.0",

test/test.js

@@ -26785,6 +26785,8 @@
   if (!document) {
     QUnit.config.noglobals = true;
     QUnit.load();
-    QUnit.start();
+    if (!QUnit.init) {
+      QUnit.start();
+    }
   }
 }.call(this));