[3.14] gh-141174: Test re-stringifying a ForwardRef (GH-155689) (#155708)

Test that re-evaluating a `ForwardRef` with `STRING` format uses the cache
(cherry picked from commit 5dab490)

Co-authored-by: dr-carlos <77367421+dr-carlos@users.noreply.github.com>

Author: miss-islington

Lib/test/test_annotationlib.py

@@ -2133,8 +2133,13 @@ def f(a: unknown | str | int | list[str] | tuple[int, ...]): ...
         self.assertIsNone(fr.__resolved_str_cache__)
 
         self.assertEqual(fr.evaluate(format=Format.STRING), "unknown | str | int | list[str] | tuple[int, ...]")
+
+        # Test that the cache is now set correctly
         self.assertEqual(fr.__resolved_str_cache__, "unknown | str | int | list[str] | tuple[int, ...]")
 
+        # Test that future evaluations return the same cache
+        self.assertIs(fr.evaluate(format=Format.STRING), fr.__resolved_str_cache__)
+
     def test_evaluate_forwardref_format(self):
         fr = ForwardRef("undef")
         evaluated = fr.evaluate(format=Format.FORWARDREF)