Mark failing test on macOS

Lib/test/test_sysconfig.py

@@ -391,6 +391,8 @@ class TestSysConfig(unittest.TestCase):
             else: # 8 byte pointer size
                 self.assertTrue(suffix.endswith('x86_64-linux-gnu.so'), suffix)
 
+    # TODO: RUSTPYTHON
+    @unitest.expectedFailure
     @unittest.skipUnless(sys.platform == 'darwin', 'OS X-specific test')
     def test_osx_ext_suffix(self):
         suffix = sysconfig.get_config_var('EXT_SUFFIX')