Don't run tests until the page is under SW control

[jeffposnick]

R: @gauntface @wibblymat @addyosmani @ebidel

As it turns out, @guantface ran into a similar race condition with the .ready promise. Switching from it to explicitly waiting on navigator.serviceWorker.controller seems to clear up the lingering test flakiness we were seeing.

We'll be following up on the .ready promise flakiness out-of-band.

Closes #37

[jeffposnick]

So this actually isn't a bug in .ready. I'd been assuming that .ready wouldn't resolve until the page was controlled, but that's not the case—the clients.claim() inside the SW's activate handler isn't enough to ensure that. See w3c/ServiceWorker#799

I'm going to add in some additional changes to this PR to disabuse myself of that assumption throughout all the tests, not just that one specific one that was failing.

[jeffposnick]

I've moved all the tests over to use a window._controlledPromise helper that I put together, to make sure that they all wait until the page is controlled before executing. That seems like the cleanest approach for the time being.

On a happy note, with #85 merged, all the tests are passing on Firefox Nightly as well!

[gauntface]

LGTM.

This makes sense to me and I'll like use this code in the sw-unit-test repo - hopefully won't make a difference but it's an important distinction.