Runner hangs when promise rejects with undefined error

[cowboyd]

The following test script causes the test suite to hang rather than fail. It's not a common case for sure, but it is useful can happen if you are using a placeholder temporarily while you can generate a failing test case:

export default test('naked rejection')
  .assert('a pure rejection with no error value', () => Promise.reject());

This should fail in some way, but instead it hangs completely.