Refactoring OptimisticConcurrencyTest

  * Sharing the service provider
  * Creating the TestStore in the constructor and disposing in Dispose()
  * Using the same model for the same fixture
  * Moving OnModelCreating to the fixture and removing provider-specific contexts

Also fixing some broken tests
One of the things you get with a derived context with DbSets is that entity types are included in the model by convention. This means you often don't have to call OnModelCreating at all for models that are built by conventions. With this pattern you will have to call OnModelCreating always. It would be nice if that didn't have to be the case.