[3.12] gh-115285: Fix test_dataclasses with -OO mode (GH-115286) (#115359)

gh-115285: Fix `test_dataclasses` with `-OO` mode (GH-115286)
(cherry picked from commit 4297d73)

Co-authored-by: Nikita Sobolev <[email protected]>

Author: miss-islington

Lib/test/test_dataclasses/__init__.py

@@ -22,6 +22,8 @@
 import typing       # Needed for the string "typing.ClassVar[int]" to work as an annotation.
 import dataclasses  # Needed for the string "dataclasses.InitVar[int]" to work as an annotation.
 
+from test import support
+
 # Just any custom exception we can catch.
 class CustomError(Exception): pass
 
@@ -2216,6 +2218,7 @@ def assertDocStrEqual(self, a, b):
         #  whitespace stripped.
         self.assertEqual(a.replace(' ', ''), b.replace(' ', ''))
 
+    @support.requires_docstrings
     def test_existing_docstring_not_overridden(self):
         @dataclass
         class C: