test: add hasAndBelongsToMany integration test

* it allows to find related object via URL scope
  GET /api/categories/{cat-id}/products

* it allows to find related objects via where filter
  GET /api/products?filter[where][categoryId]={cat-id}
  (skipped for now)

* it includes requested related models in `find`
  GET /api/categories/findOne
      ?filter[where][id]=CAT-ID&filter[include]=products

* it includes requested related models in `findById`
  GET /api/categories/{cat-id}?include=products
  (skipped for now)