Retrying execution strategy in tests hides MARS requirement for split query #21427

When working on the EFCore.PG preview6 release, I ran into something curious: all tests were passing although I did nothing to enforce buffering for split queries (as discussed in #21420). Long story short, it turns out that like for SQL Server, I have an NpgsqlDbContextOptionsBuilderExtensions which configures a retrying strategy for all tests. That strategy turns on buffering, which makes query splitting tests pass; but a user trying split queries would get failures because of MARS.

Our functional test suite should fail if query splitting isn't properly supported. Options:

• Find out why we use a retrying strategy everywhere and consider removing it. This would make our tests run more similar to how many users use EF Core.
• If we need to keep the strategy (e.g. because otherwise tests are flaky), we should at least add some testing without the strategy the would fail if the provider hasn't been properly implemented split queries.