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
I would agree if we needed fixtures in all tests. Using a fixture that doesn't have any state is misleading.

Another benefit of using derived contexts is that it's closer to the typical use case of customizing the model by overriding OnModelCreating.