Adjust "no arbiter" test for new runtime defaults

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`.

tests/test_local.py

@@ -11,15 +11,15 @@ from conftest import tractor_test
 
 
 @pytest.mark.trio
-async def test_no_arbitter():
+async def test_no_runtime():
     """An arbitter must be established before any nurseries
     can be created.
 
     (In other words ``tractor.open_root_actor()`` must be engaged at
     some point?)
     """
-    with pytest.raises(RuntimeError):
+    with pytest.raises(RuntimeError) :
-        with tractor.open_nursery():
+        async with tractor.find_actor('doggy'):
             pass