Improve _update_list_field test method

[vkrasnovyd]

As mentioned in #3014, the following changes should be made to _update_list_field() method:

• Rewrite with setting list_fields in the Event;
• Move from tests/system/action/motion/test_create_forwarded.py to BaseActionTestCase next to update_model. Suggested name: update_model_list_fields.

[hjanott]

A similar method was created in #3175 named get_update_list_events and can be applied to the case in #3014. I would still vote for another method with the name in the description.