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
@ajcvickers, assuming that no two fixtures share a database. E.g. NorthwindQueryFixture and MappingQueryFixture both use the Northwind database. This is (was?) handled by concurrency protection in concrete TestDatabase class.