Run each test in its own <iframe>

This is not blocking the initial launch, so feel free to put it on the back-burner for now.

The Jasmine test harness still runs in the parent window and reports to PhantomJS via `window.callPhantom`, but each test `<iframe>` has its own copy of `react-test.js` and each individual test module is required in the global context of a separate `<iframe>`.

This gives us a significant approximation of the benefits of mocking, at least in terms of isolating tests from one another.

cr @jeffmo @zpao