Add an `actorc` test-driven-dev suite

Defining how an actor-nursery should emit an eg based on non-graceful
cancellation in a new `test_actor_nursery` module. Obviously fails atm
until the implementation is completed.

tests/test_actor_nursery.py

@@ -0,0 +1,98 @@
+'''
+Basic `ActorNursery` operations and closure semantics,
+- basic remote error collection,
+- basic multi-subactor cancellation.
+
+'''
+# import os
+# import signal
+# import platform
+# import time
+# from itertools import repeat
+
+import pytest
+import trio
+import tractor
+from tractor._exceptions import ActorCancelled
+# from tractor._testing import (
+#     tractor_test,
+# )
+# from .conftest import no_windows
+
+
+@pytest.mark.parametrize(
+    'num_subs',
+    [
+        1,
+        3,
+    ]
+)
+def test_one_cancels_all(
+    start_method: str,
+    loglevel: str,
+    debug_mode: bool,
+    num_subs: int,
+):
+    '''
+    Verify that ifa a single error bubbles to the an-scope the
+    nursery will be cancelled (just like in `trio`); this is a
+    one-cancels-all style strategy and are only supervision policy
+    at the moment.
+
+    '''
+    async def main():
+        try:
+            rte = RuntimeError('Uh oh something bad in parent')
+            async with tractor.open_nursery(
+                start_method=start_method,
+                loglevel=loglevel,
+                debug_mode=debug_mode,
+            ) as an:
+
+                # spawn the same number of deamon actors which should be cancelled
+                dactor_portals = []
+                for i in range(num_subs):
+                    name: str= f'sub_{i}'
+                    ptl: tractor.Portal = await an.start_actor(
+                        name=name,
+                        enable_modules=[__name__],
+                    )
+                    dactor_portals.append(ptl)
+
+                    # wait for booted
+                    async with tractor.wait_for_actor(name):
+                        print(f'{name!r} is up.')
+
+                # simulate uncaught exc
+                raise rte
+
+            # should error here with a ``RemoteActorError`` or ``MultiError``
+
+        except BaseExceptionGroup as _beg:
+            beg = _beg
+
+            # ?TODO? why can't we do `is` on beg?
+            assert (
+                beg.exceptions
+                ==
+                an.maybe_error.exceptions
+            )
+
+            assert len(beg.exceptions) == (
+                num_subs
+                +
+                1  # rte from root
+            )
+
+            # all subactors should have been implicitly
+            # `Portal.cancel_actor()`ed.
+            excs = list(beg.exceptions)
+            excs.remove(rte)
+            for exc in excs:
+                assert isinstance(exc, ActorCancelled)
+
+            assert an._scope_error is rte
+            assert not an._children
+            assert an.cancelled is True
+
+    trio.run(main)