Add a basic test of guest mode

trio/_core/tests/test_guest_mode.py

@@ -0,0 +1,50 @@
+import pytest
+import asyncio
+
+import trio
+
+def test_guest_mode_basic():
+    async def aio_main():
+        loop = asyncio.get_running_loop()
+
+        trio_done_fut = asyncio.Future()
+        def trio_done_callback(main_outcome):
+            print(f"trio_main finished: {main_outcome!r}")
+            trio_done_fut.set_result(main_outcome)
+
+        trio.lowlevel.start_guest_run(
+            trio_main,
+            run_sync_soon_threadsafe=loop.call_soon_threadsafe,
+            done_callback=trio_done_callback,
+        )
+
+        return (await trio_done_fut).unwrap()
+
+    async def trio_main():
+        print("trio_main!")
+
+        to_trio, from_aio = trio.open_memory_channel(float("inf"))
+        from_trio = asyncio.Queue()
+
+        aio_task = asyncio.create_task(aio_pingpong(from_trio, to_trio))
+
+        from_trio.put_nowait(0)
+
+        async for n in from_aio:
+            print(f"trio got: {n}")
+            from_trio.put_nowait(n + 1)
+            if n >= 10:
+                aio_task.cancel()
+                return "trio-main-done"
+
+    async def aio_pingpong(from_trio, to_trio):
+        print("aio_pingpong!")
+
+        while True:
+            n = await from_trio.get()
+            print(f"aio got: {n}")
+            to_trio.send_nowait(n + 1)
+
+    loop = asyncio.new_event_loop()
+    assert loop.run_until_complete(aio_main()) == "trio-main-done"
+    loop.close()