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.
this can just use `return this.session.executeAsync`, there is no reason to generate a new Command object