Fixes #164 - add initial guide on testing elements

reviewers: @ebidel @robdodson 

Preview: https://github.com/Polymer/docs/blob/4b8e748dad6007bc859ae057ba6f20610fa25a0a/articles/unit-testing-elements.md

I plan on following this up later in Q4 with a more detailed end-to-end walkthrough, but this will at least give us something until then. Go go testing!

Beautiful

On Fri, Oct 31, 2014, 08:11 Addy Osmani <notifications@github.com> wrote:

> In articles/unit-testing-elements.md:
>
> > +              selectEventCounter++;
> > +              assert.equal(e.detail.item, s.selectedItem);
> > +            }
> > +      });
> > +    });
> > +
> > +Great. Now to set the `selected` item in the list to "2" we can write:
> > +
> > +    test('if core-select is fired each time a different item in a list is selected', function() {
> > +      // ...
> > +      s.selected = 2;
> > +    });
> > +
> > +Which will trigger the "core-select" event to be fired.
> > +
> > +**Note:** In Polymer’s unit tests, just to ensure that all of our bindings are correctly getting updated when we dynamically change values in this way, we can call `Platform.flush()`:
>
> "flush() allows us to asynchronously dirty check pending objects are
> observed and ensures notification callbacks are dispatched accordingly. It
> also triggers a flush of any pending events."
>
> Something like that?
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/Polymer/docs/pull/650/files#r19671960>.
>