Fix async test helpers

[teddyzeenny]

Refactored async helpers to be more robust and fixed some issues with the previous implementation. Before now you couldn't block an async helper by returning any promise.

This didn't work (but now does):

Ember.Test.registerAsyncHelper('sleep', function(app, duration) {
  return new Ember.RSVP.Promise(function(resolve) {
    setTimeout(resolve, duration);
  });
});

The only two ways that worked were returning wait or using Ember.Test.promise which shouldn't be public api.

Ember.Test.registerAsyncHelper('sleep', function(app, duration) {
  let promise = new Ember.RSVP.Promise(function(resolve) {
    setTimeout(resolve, duration);
  });
  return wait(promise);
});

Ember.Test.registerAsyncHelper('sleep', function(app, duration) {
  return Ember.Test.promise(function(resolve) {
    setTimeout(resolve, duration);
  });
});

Some async helpers were also accidentally triggered synchronously if they were the first helper in a test. People should not rely on this behavior, so now all async helpers are triggered asynchronously. (The checkbox helpers were relying on this unwanted behavior so I fixed them).

[rwjblue]

LGTM

[seanpdoyle]

@teddyzeenny 👏