As per Maik's suggestion, move 'change_order_statuses' from order_actions to actions
Make input for order action a visible checkbox, so that there is no conflict with the 'Download CSV' action. This is not pretty, but it works.
Change name of action in checkbox to update order status from 'order_action' to 'action'. Finally this allows the user to download selected or change the status (now there is no conflict)
Try to change reload_page_response() to get_success_url(), but changed the name of the function only. Will try later to change the actual function.
Refactor so that updating order status is handled by update_order_statuses() rather than post(). Need to fix test. Need to make changes cascade to line items.
Need to figure out how to check the checkbox for the 'Change Status' checkbox, so that the test can run properly.