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`.