Build: Support Node.js export parity with CommonJS

Ref #521

More info: https://github.com/jquery/qunit/issues/521#issuecomment-65656852
actually, just one of the 3 asserts above is fine, maybe:

```js
assert.ok(
  qunit instanceof QUnit.constructor,
  "Required module QUnit built from its own constructor"
);
```

*using the global QUnit.constructor*