[RFC] Clean up/rethink test infrastructure

[beauby]

Currently the tests use models and serializers defined in test/fixtures/poro.rb and test/fixtures/active_record.rb (as well as some dynamically defined/locally scoped serializers inside tests) in combination with actual fixtures defined in the setup method of many test classes, which leads to lots of code duplication.
I am opening this RFC to discuss alternatives to this, as I feel writing tests should be easier and cleaner.

[NullVoxPopuli]

I've always been a fan of having a support folder under my test directory, and then having files named after groups of what they represent. I guess in our case, it could be: models.rb, serializers.rb, model_support.rb (this one could be the model object itself) etc.

[bf4]

ref something I wrote in RSpec on 0.10.0.rc1, #878 (comment) as I will (hopefully) get to in #967