[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`.
Merge pull request #4203 from rjackson/fix_lazy_routing

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