Fixup test-helpers/index.

* Use `asserts` instead of `assertions` (that is what is used internally in the helpers).
* Make `callForEach` a function generator (so that `this.asserts`
  works).
* Fix typo in `Composite` prototype.
* MOAR TESTS!!
Merge pull request #136 from rwjblue/fix-grouped-helper

Fixup test-helpers/index.