Use mock's call assert methods over call_args_list

The way we're checking the value of call_args_list on a mock method is
essentially what assert_called_once_with() since we also know the
method is only called once. In other cases, we can use
assert_has_calls().

I'm slightly nervous of using call_args_list because it's the kind of
thing you could imagine changing in future, even though it's not
marked private.

Change-Id: I7eb3d095f7a1ff9a4abc9e027193d4918d0cda6c
Merge "Use mock's call assert methods over call_args_list"