Refactoring Northwind and OptimisticConcurrency functional tests

Split Northwind model into separate files and added a context class
Avoiding sharing any state between the contexts that mutate the model or the data
Renamed provider-specific test classes to include the provider name
Cleaning leftover databases
The fixtures are just for perf benefit of sharing read-only instances. Some tests, like OptimisticConcurrencyTest do not have anything that can be shared and thus don't use fixtures.

It would be beneficial to test sharing the same model between different providers, but then the FunctionalTest project that contains the models would have to reference the providers and become provider-inclusive instead of provider-agnostic.