capture stderr to silence output during test_coroutines (closes #27968)

Lib/test/test_coroutines.py

@@ -1568,10 +1568,11 @@ class CoroutineTest(unittest.TestCase):
     def test_fatal_coro_warning(self):
         # Issue 27811
         async def func(): pass
-        with warnings.catch_warnings():
+        with warnings.catch_warnings(), support.captured_stderr() as stderr:
             warnings.filterwarnings("error")
             func()
             support.gc_collect()
+        self.assertIn("was never awaited", stderr.getvalue())
 
 
 class CoroAsyncIOCompatTest(unittest.TestCase):