[Abstract protocol] Make tests work with gRPC enabled and disabled

This requires depends on the following issue to be shipped first:

• https://github.com/datafusion-contrib/datafusion-distributed/issues/576

Some of the current tests assume that there's a gRPC implementation wired up in the SessionContext, and will attempt to communicate between workers using gRPC, either through in-memory duplex channels or localhost.

This is mostly driven by the test harness shipped in test_utils/. However, once we have a full in-memory implementation of the WorkerChannel, we could be wiring it up to the test utils and run tests with fully in-memory comms.

I think there's still value in being able to run the tests with the gRPC implementation, so the best direction that comes to mind is:

1. Scope the gRPC-specific test utils under the grpc/ folder.
2. In the central test_utils/ go to every place that builds a distributed-capable SessionContext and switch between in-memory or gRPC WorkerChannel implementations based on whether the grpc flag is enabled or not.

First review issue #576, then inspect the gRPC-specific helpers under grpc/ and the distributed-capable SessionContext construction in test_utils/. Run the test suite with the grpc flag enabled and disabled to identify assumptions about WorkerChannel implementations. Done means both configurations use the appropriate in-memory or gRPC channel and the tests pass.