Basic tests for array.js.  Don't merge this yet.

I've added a couple of tests for array.js, I'll need to add a lot more, especially for invalid inputs and stuff. This PR is just to get feedback and facilitate discussion around what sort of failure strategies we want and what kind of tests should be written and possibly, what features need to be added/removed to array.js.
would be nice if we had `array.equivalent(a,b)` to return true if the elements are all the same. But of course, the question of whether it is a shallow comparison / deep comparison is a big one. All because we have an assignment statement in JavaScript. :smiling_imp: