Mark failing test in test_fstring

Lib/test/test_fstring.py

@@ -1526,6 +1526,7 @@ x = (
                              r"f'{1000:j}'",
                             ])
 
+    @unittest.expectedFailure  # TODO: RUSTPYTHON
     def test_filename_in_syntaxerror(self):
         # see issue 38964
         with temp_cwd() as cwd: