Turns out this test was being silently ignored due to incorrect usage of
sync opening of our `.open_nursery()` block (with a `with` not `async
with`) and thus was an noop XD

Instead this fixes the test to call a `tractor` discovery built-in
without starting the runtime (which is now done implicitly when a user
opens a nursery) which should result in the prior expected outcome,
a `RuntimeError`.

Pretty sure this is the final touch to alleviate all our debug lock
headaches! Instead of trying to revert to the "last" handler (as `pdb`
does internally in the stdlib) we always just revert to the handler
`trio` registers during startup. Further this seems to allow cancelling
the root-side locking task if it's detected as stale IFF we only do this
when the root actor is in a "no more IPC peers" state.

Deatz:
- (always) set `._debug.Lock._trio_handler` as the `trio` version, not
  some last used handler to make sure we're getting the ctrl-c handling
  we want when not in debug mode.
- assign the trio handler in `open_root_actor()`
  `._runtime._async_main()` to be sure it's applied in subactors as well
  as the root.
- only do debug lock blocking and root-side-locking-task cancels when
  a "no peers" condition is detected in the root actor: i.e. no IPC
  channels are detected by the root meaning it's impossible any actor
  has a sane lock-state ongoing for debug mode.

If we pack the nursery parent task's error into the `errors` table
directly in the handler, we don't need to specially handle packing that
same error into any exception group raised while handling sub-actor
cancellation; drops some ugly indentation ;)