Fix tests when run with -Werror

Author: vstinner

Lib/test/test_warnings/__init__.py

@@ -518,14 +518,18 @@ def custom_showwarning(message, category, filename, lineno,
                                file=None, line=None):
             warns.append(message)
 
-        with support.swap_attr(self.module, 'showwarning', custom_showwarning):
-            with self.module.catch_warnings(record=True) as recorded:
-                self.module.warn("recorded")
-            self.assertEqual(len(recorded), 1)
-            self.assertEqual(str(recorded[0].message), 'recorded')
-            self.assertIs(self.module.showwarning, custom_showwarning)
-
-            self.module.warn("custom")
+        with self.module.catch_warnings():
+            self.module.resetwarnings()
+
+            with support.swap_attr(self.module, 'showwarning',
+                                   custom_showwarning):
+                with self.module.catch_warnings(record=True) as recorded:
+                    self.module.warn("recorded")
+                self.assertEqual(len(recorded), 1)
+                self.assertEqual(str(recorded[0].message), 'recorded')
+                self.assertIs(self.module.showwarning, custom_showwarning)
+
+                self.module.warn("custom")
 
         self.assertEqual(len(warns), 1)
         self.assertEqual(str(warns[0]), "custom")