Improve Testfrozen.test_frozen

Author: Prometheus3375

Lib/test/test_dataclasses/__init__.py

@@ -3068,6 +3068,9 @@ class C:
                 with self.assertRaises(FrozenInstanceError):
                     c.i = 5
                 self.assertEqual(c.i, 10)
+                with self.assertRaises(FrozenInstanceError):
+                    del c.i
+                self.assertEqual(c.i, 10)
 
     def test_frozen_empty(self):
         for slots in (False, True):