Enhance resize test

Author: vstinner

Lib/test/test_capi/test_unicode.py

@@ -224,7 +224,7 @@ def resize(s, length, new=True, compute_hash=False):
             else:
                 # In-size replace can return the same address, or not.
                 # So 'is_new_obj' cannot be tested.
-                self.assertFalse(sys._is_immortal(result))
+                self.assert_is_mutable(result, refcnt)
 
             return result