The reason for this "duplication" with the `--asyncio` CLI flag (passed
to the child during spawn) is 2-fold:
- allows verifying inside `Actor._from_parent()` that the `trio` runtime was
started via `.start_guest_run()` as well as if the
`Actor._infected_aio` spawn-entrypoint value has been set (by the
`._entry.<spawn-backend>_main()` whenever `--asyncio` is passed)
such that any mismatch can be signaled via an `InternalError`.
- enables checking the `._state._runtime_vars['_is_infected_aio']` value
directly (say from a non-actor/`trio`-thread) instead of calling
`._state.current_actor(err_on_no_runtime=False)` in certain edge
cases.
Impl/testing deats:
- add `._state._runtime_vars['_is_infected_aio'] = False` default.
- raise `InternalError` on any `--asyncio`-flag-passed vs.
`_runtime_vars`-value-relayed-from-parent inside
`Actor._from_parent()` and include a `Runner.is_guest` assert for good
measure B)
- set and relay `infect_asyncio: bool` via runtime-vars to child in
`ActorNursery.start_actor()`.
- verify `actor.is_infected_aio()`, `actor._infected_aio` and
`_state._runtime_vars['_is_infected_aio']` are all set in test suite's
`asyncio_actor()` endpoint.
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