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
I know it's not really part of the change you are making, but the pattern of setting the partition and row key to define a primary key definition and then immediately replacing the primary key definition with a different one seems problematic since the store thinks items have one identity while the EF thinks they have a different identity. I think it would be interesting to consider the relationship between identity and partition and row key in ATS, and also the implications of trying to reuse the same tests for different providers when the model is different for those providers.