Consider offering a test-probe like abstraction for testing DA types

We technically are able to create mocks using the DA by resolving and using a "mock" rather than "remote" call infra.
We could then assert and mock replies more easily.

This came up because we lost this ability from moving from pure message based actor-refs to method based distributed actor types, so porting some tests becomes a bit of a challange.

Start by reviewing the existing distributed actor testing approach and the mock-versus-remote call infrastructure described in the issue, then examine tests being ported from message-based actor references. Done means a test-probe-like abstraction can create mocks, provide mock replies, and support straightforward assertions for distributed actor types.

Written by the indexing model from the issue text.