Converted dojo/window viewport dimension tests to intern.

The dojo/window tests are going to be tough to convert. I would like some feedback before proceeding with conversion of all the dojo/window doh tests. 

I did the conversion with as a little change to the original doh tests as possible. 

Here is a link to the original tests for comparison. https://github.com/dojo/dojo/blob/master/tests/window/viewport.html
I found a better way to do this:

https://github.com/bryanforbes/dojo/blob/intern-conversion/tests-intern/functional/behavior.html#L37-L53
https://github.com/bryanforbes/dojo/blob/intern-conversion/tests-intern/functional/behavior.js#L12-L16

When using `waitForCondition()`, the test runner will poll the selenium server over the wire until it is finished. Using `executeAsync()` allows for the test page to signal the test runner when it's ready.