Implement `(allow|expect)_next_found_instance_of` mock helpers

Since ActiveRecord is instantiating the objects directly by calling
`.allocate` on the model classes, mocking with `next_instance_of` mock
helpers is not possible as it is depending on the `.new` method being
called. To make it possible to mock next instance of ActiveRecord
models, we had to implement a new set of mock helpers which rely on
the `.allocate` method to be called.