I'm successful to add a sub.expectedFailure() to subTest, but in current API, it isn't that will done.

First, you will not get any alarm or information that there is a  test been treat as "expected failure" at the result text.

Second, it just act like you putting @unittest.expectedFailure at the top of the test.

Third, current result didn't put expected failure as a result category, so it will not show it at the result if we skip some test in the subTest, and it will need to changed to add an expected failure list to handle multiple expected failure in a test.