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
By using fixtures only when necessary we make it more explicit what tests are safe to share state. I've only ran into caching issues when sharing the service provider between mutating tests and this is an easy mistake to make if they use a fixture to setup the service provider. I believe that in xUnit.net the fixtures _are_ only about sharing data.

IMHO having provider-specific code to create the model outside the context is even further from what we advocate.