Move over relevant test from the "context semantics" test module which
was already verifying peer-caused-`ContextCancelled.canceller: tuple`
error info and propagation during an inter-peer cancellation scenario.
Also begin a more general set of inter-peer cancellation tests starting
with the simplest case where when a peer is cancelled the parent should
NOT get an "muted" `trio.Cancelled` and instead
a `tractor.ContextCancelled` with a `.canceller: tuple` which points to
the sibling actor which requested the peer cancel.
Tyler Goodlet