Fix incorrect usage of `app` in app.test.js

The `beforeEach` hook was using a local `var app`, the test was
accessing global `app` created by `test/support.js`.
Make app.models unique per app instance

Remove a source of confusion in unit-tests.
Merge pull request #168 from strongloop/feature/new-model-registry-per-app

Make app.models unique per app instance