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
Currently yes, but I think that set discovery should be a convention on ModelBuilder instead of being a responsobility of ModelSource.