As per a lot of the recent refinements to `Context` cancellation, add
a new test case to replicate the original hang-on-cancel found with
`modden` when using a client actor to spawn a subactor in some other
tree where despite `Context.cancel()` being called the requesting client
would hang on the opened context with the server.
The specific scenario added here is to have,
- root actor spawns 2 children: a client and a spawn server.
- the spawn server opens with a spawn-request serve loop and begins to
wait for the client.
- client spawns and connects to the sibling spawn server, requests to
spawn a sub-actor, the "little bro", connects to it then does some
echo streaming, cancels the request with it's sibling (the spawn
server) which should in turn cancel the root's-grandchild and result
in a cancel-ack back to the client's `.open_context()`.
- root ensures that it can also connect to the grandchild (little bro),
do the same echo streaming, then ensure everything tears down
correctly after cancelling all the children.
More refinements to come here obvi in the specific cancellation
semantics and possibly causes.
Also tweaks the other tests in suite to use the new `Context` properties
recently introduced and similarly updated in the previous patch to the
ctx-semantics suite.
cfcbc4da01