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
@anpete That's still accurate
In the future we should use ICollectionFixture that has the desired behavior, however it currently doesn't work.