Add a first serious inter-peer remote cancel suite

Tests that appropriate `Context` exit state, the relay of
a `ContextCancelled` error and its `.canceller: tuple[str, str]` value
are set when an inter-peer cancellation happens via an "out of band"
request method (in this case using `Portal.cancel_actor()` and that
cancellation is propagated "horizontally" to other peers. Verify that
any such cancellation scenario which also experiences an "error during
`ContextCancelled` handling" DOES NOT result in that further error being
suppressed and that the user's exception bubbles out of the
`Context.open_context()` block(s) appropriately!

Likely more tests to come as well as some factoring of the teardown
state checks where possible.

Pertains to serious testing the major work landing in #357