This reverts commit e8cab2d34a.
reason: this commit was accidentally added. I wanted to test that the helpers are working inside mouted engine on the fake app, but it turned out that this test is not really testing that...
extract model definitions for each ORM under spec/fake_app, and load them only if target ORM is loaded
by this mechanism, all ORMs share a model name "User", and the other specs such as helper specs and request specs will run polymorphically