add getDijit promise helper, and test case

This adds a promise helper to simplify the boilerplate for writing Dijit functional tests.

I wasn't sure how to properly write the test for this without pulling in Dijit, so I wrote the initial test against the CDN which is probably wrong, so I could use guidance on that. In that context, I also wasn't sure what the appropriate values would be for setting timeouts, so I could use guidance there as well.
isn't this more or less redundant with something like:

```js
var widget = command.then(dijitHelper.get(widgetId));
var value = widget.then(function (widget) {
  return widget.get('property');
});
```