Ready event seems to be unreliable

[lpeabody]

What are some scenarios where the ready event always fires but some scenarios still log the following:

ReadyEvent not detected within readyTimeout limit. (30000 ms) ...

It's inconsistent, so it seems like maybe the ready event fires too quickly, perhaps?

[lpeabody]

I'm unsure just by reading the source, but, it appears that BackstopTools need to be installed before the ready event is logged to the console?

I'm finding this to be helpful:

// Let's promise to resolve that BackstopTools is installed before outputting the ready event to the console.
let backstopToolsInstalledResolve;
const backstopToolsInstalledPromise = new Promise(resolve => {
  backstopToolsInstalledResolve = resolve;
});
const backstopReadyWhenAllResolved = [
  backstopToolsInstalledPromise
];

...

// Give it up to 10 seconds to become ready.
let readyEventIntervalCounter = 0;
const readyEventInterval = setInterval(() => {
  if (window._readyEvent) {
    backstopToolsInstalledResolve();
    clearInterval(readyEventInterval);
  }
  if (readyEventIntervalCounter++ > 10) {
    clearInterval(readyEventInterval);
  }
}, 1000);

// When all required promises to be ready have been fulfilled, log the event.
// Also wait a few seconds for good measure.
Promise.all(backstopReadyWhenAllResolved)
  .then(() => {
    setTimeout(() => {
      console.log(window._readyEvent);
    }, 3000);
  });

I think it would be helpful to suggest this as a recipe in the documentation, otherwise using the ready event feature may seem like it's not working as intended and its not obvious at all as to why.

EDIT: If you think it would be useful I can setup a PR with the added documentation.