bpo-33912: Fix test_warnings when run with -Werror (GH-7839)

Add missing warning filter to test_exec_filename().

Lib/test/test_warnings/__init__.py

@@ -447,6 +447,7 @@ class WarnTests(BaseTest):
                            "warnings.warn('hello', UserWarning)"),
                           filename, "exec")
         with original_warnings.catch_warnings(record=True) as w:
+            self.module.simplefilter("always", category=UserWarning)
             exec(codeobj)
         self.assertEqual(w[0].filename, filename)