Splitting DbSet operations and SaveChanges in perf tests

Changes:
* Scoping collection in UpdatePipeline tests to just cover SaveChanges
* Adding tests that target DbSet.Add/Remove/Attach/Update
We talked about that but it seemed to add little value since there is a dedicated database per set of tests and we only need to create it if it hasn't already been created (since the test data needs to be static in order to have comparable runs).