test(signal): add sleep to ensure shutdown completion (#2427)

tests/supervisors/test_signal.py

@@ -40,6 +40,7 @@ async def wait_app(scope, receive, send):
             # ensure httpx has processed the response and result is complete
             await req
             assert req.result().status_code == 200
+            await asyncio.sleep(0.1)  # ensure shutdown is complete
 
 
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