mock.assert_called_once() is not a valid method

mock.assert_called_once() is a no-op that tests nothing.  Instead
with mock.assert_called_once_with() should be used (or use
assertEqual(1, mock_obj.call_count) if you don't want to check
parameters).

Change-Id: Ib302f2e888306558535cdd3e5edf447809aee430
Partial-Bug: #1365751