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
That's an interesting idea. In the old stack some people had quite strong opinions that the model builder should not be coupled in any way to the context. But maybe it can be done in an abstract enough way that the coupling is avoided. Also, when would the convention run? I.e. Would the entity types already be in the model when OnModelCreating is run? I would file an issue on this unless you are already tracking it as part of the convention work.