[BUGFIX beta] Allow test helpers to be injected on specific object.

This allows the test helpers to be injected into an object other than
`window`.
Merge pull request #3686 from rjackson/refactor_testing

[BUGFIX beta] Allow test helpers to be injected on specific object.
Remove inconsistent global reference to Handlebars in helper
Merge pull request #3688 from kumavis/master

Fix typo in Handlebars
[FEATURE ember-testing-lazy-routing]

* `setupForTesting` uses an initializer to defer readiness (which makes
`App.reset()` honor the standard `setupForTesting` deferred state.
* `visit` helper has been updated to advance readiness (via `boot`
  helper) upon first call.