Testing helper fixes for the browser build

This fixes a couple errors preventing the testing helpers from working in the browser build:

- There was a missing module dependency: `simple-auth/configuration`
- The testing helpers are contained in the `simple-auth-testing/test-helpers` module, not the two modules listed currently
- The `simple-auth-testing/test-helpers` module has a dependency on `global.Ember.Test`, not `global.Ember.testing` (which isn't created until after calling `App.setupForTesting()`)