Simplify the fix.

Author: serhiy-storchaka

Lib/test/test_unittest/test_case.py

@@ -346,11 +346,10 @@ class Foo(unittest.TestCase):
             async def test1(self):
                 return 1
 
-        with warnings.catch_warnings():
-            # Ignore RuntimeWarning: coroutine '...' was never awaited
-            warnings.simplefilter("ignore", RuntimeWarning)
-            with self.assertWarns((DeprecationWarning, )) as w:
-                Foo('test1').run()
+        with self.assertWarns(DeprecationWarning) as w:
+            warnings.filterwarnings('ignore',
+                    'coroutine .* was never awaited', RuntimeWarning)
+            Foo('test1').run()
             support.gc_collect()
         self.assertIn('It is deprecated to return a value that is not None', str(w.warning))
         self.assertIn('test1', str(w.warning))