fixed a couple tests

made a couple fixes to the tests

- the let test for non-mozilla browsers wasn't running because the variable being used to store the results of part 1 was undefined not falsey
- same with yield except I had to add an actual test (copy and paste) to the callback.
- iterable was using out of date syntax (source [this post](http://domenic.me/2013/09/06/es6-iterators-generators-and-iterables/) by @domenic)
- WeakMap now has a comparable test to Map based on its capabilities.