Tests for khan-exercises utilities

yo @beneater here's what i got going on right now. This isn't ready to merge yet, so if you can take a look and give me some thoughts, that'd be good. The implementation of KhanUtil.load in the tests page has the collision detection but the one in `khan-exercises` retains the old behavior. 

in particular, the [angles test](https://github.com/Khan/khan-exercises/pull/new/tests#diff-7) is the most like what we'd be asking people to do. Either fill in the exercise name/property as a blank stub function or include some assertions in there.

rock and/or roll