Starts with some very basic cases:
- verify both subactor-as-child-ctx-task send side validation (failures)
as well as relay and raise on root-parent-side-task.
- wrap failure expectation cases that bubble out of `@acm`s with
a `maybe_expect_raises()` equiv wrapper with an embedded timeout.
- add `Return` cases including invalid by `str` and valid by a `None`.
Still ToDo:
- commit impl changes to make the bulk of this suite pass.
- adjust how `MsgTypeError`s format the local (`.started()`) send side
`.tb_str` such that we don't do a "boxed" error prior to
`pack_error()` being called normally prior to `Error` transit.
Tyler Goodlet