Check `ReactChildren.{map, forEach}` callback argument

Fixes #1755

/cc @spicyj
I thought about that but then saw that other files did it like this: https://github.com/facebook/react/blob/94fd726f86a36cb682d9680a67df544a0a889e53/src/browser/ui/dom/components/__tests__/ReactDOMInput-test.js#L24-L36

Seems like only the modules/functions under test are put in `beforeEach`. Either way, let me know which one you prefer.