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
"The fixtures are just for perf benefit of sharing read-only instances. Some tests, like OptimisticConcurrencyTest do not have anything that can be shared and thus don't use fixtures."

Fixtures are a way to set up the "context" that one or more tests depend on. Given we are using them already, with the base/derived pattern, to me, it makes sense that we just standardize on a single, consistent pattern, rather than having multiple ways of setting up tests. What is the advantage of having both fixtures _and_ derived contexts?