Add test for #265: "msg sent before stream opened"

This always triggered the mentioned race condition.
We need to figure out the best approach to avoid this case.

tests/test_context_streams.py

@@ -0,0 +1,54 @@
+'''
+``async with ():`` inlined context-stream cancellation testing.
+
+Verify the we raise errors when streams are opened prior to sync-opening
+a ``tractor.Context`` beforehand.
+
+'''
+import trio
+from trio.lowlevel import current_task
+import tractor
+
+
+@tractor.context
+async def never_open_stream(
+
+    ctx:  tractor.Context,
+
+) -> None:
+    '''Bidir streaming endpoint which will stream
+    back any sequence it is sent item-wise.
+
+    '''
+    await ctx.started()
+    await trio.sleep_forever()
+
+
+def test_no_far_end_stream_opened():
+    '''
+    This should exemplify the bug from:
+    https://github.com/goodboy/tractor/issues/265
+
+    '''
+    async def main():
+        async with tractor.open_nursery() as n:
+            portal = await n.start_actor(
+                'starts_no_stream',
+                enable_modules=[__name__],
+            )
+
+            async with (
+                portal.open_context(
+                    never_open_stream,) as (ctx, sent),
+                ctx.open_stream() as stream,
+            ):
+                assert sent is None
+
+                # XXX: so the question is whether
+                # this should error if the far end
+                # has not yet called `ctx.open_stream()`?
+                # If we decide to do that we need a synchronization
+                # message which is sent from that call?
+                await stream.send('yo')
+
+    trio.run(main)