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
As I understand it from talking to @bricelam, in terms of concurrency issues when tests are being run in parallel, there will be one and only one test fixture created for each test class. I think this means that as long as there is a 1:1 relationship between test fixtures and classes, then this approach will not result in two classes attempting to initialize the database twice. But if more than one test class uses the same fixture, then there will be an issue. Do we, or can we, ensure that there is a 1:1 relationship between test class and fixture, or are concurrency issues around database initialization not a problem for some other reason?