Follow-up for https://github.com/strongloop/loopback-next/pull/5251. In order to allow users to run integration tests, bind the LB3 models to the LB4 app's context. ## Acceptance criteria - [x] Add a property `models` to `Lb3Application` interface - [x] When the LB3 application is loaded, add the models to LB4 context - [x] Add test(s) to verify