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
The thing is that there isn't really any infra, since all we need is a connection string. Creating the database etc. is all handled within the test since it deals with seed data etc. at the same time (using the product APIs).

There may be some value with x-provider tests though, we haven't really worked out how that's going to work with perf just yet as we don't want the code to deviate too much from what a customer would write in their app.