Track pending frozendict pickle support during the 3.15 transition

Keep the two existing frozendict pickle tests running as expected failures until #8951 supplies the builtin. Unexpected success will signal removal of these temporary markers.

Assisted-by: OpenAI Codex:model identifier unavailable

Lib/test/pickletester.py

@@ -4394,6 +4394,8 @@ class AbstractPickleTests:
     def test_fast_save_enter_set(self):
         self.fast_save_enter(lambda i: set([i]))
 
+    # TODO: RUSTPYTHON; frozendict support: https://github.com/RustPython/RustPython/pull/8951
+    @unittest.expectedFailure
     def test_fast_save_enter_frozendict(self):
         if self.py_version < (3, 15):
             self.skipTest('need frozendict')
@@ -4441,6 +4443,8 @@ class AbstractPickleTests:
                                 depth=FAST_NESTING_LIMIT+1,
                                 compare_equal=False)
 
+    # TODO: RUSTPYTHON; frozendict support: https://github.com/RustPython/RustPython/pull/8951
+    @unittest.expectedFailure
     def test_deep_nested_struct_frozendict(self):
         if self.py_version < (3, 15):
             self.skipTest('need frozendict')