Add ActiveRecord spy for testing mixins and configuration

This adds a `RecordSpy` for testing ActiveRecord mixins. This
is a useful tool for testing mixins that add ActiveRecord
validations, callbacks, etc. The goal is to be able to test these
mixins without actually providing an ActiveRecord model.

@kellyredding - I originally put an issue for this in MR, but I think it fits better in Ardb. I'd like to use when moving some of the record mixins in Pro to I-App.