[BUGFIX release] Fix ember-testing-lazy-routing.

The initial implementation of the `ember-testing-lazy-routing` feature 
functions by delaying application boot until after the first `visit` call.
This works well, but unfortunately, once we do the `visit` we are booting the
router (via normal `App.startRouting` call in
`didBecomeReady`) which calls `handleURL` and then triggering another
`handleURL` by invoking it directly.

To ensure that the initial `handleURL` call (done from within the application
startup process) routes to the desired URL (instead of `/`) a new `initialURL`
param has been added to the `router.startRouting`.

Resolves #4199.