Adds automated checks for product image resizing, zoom behavior, and shared image handling used by the online store. This helps reduce the risk of image display issues for shoppers and improves reliability without changing visible functionality.

This commit add tests for new mixin product.image and resizing.
Purpose is to test feature merged by commit: 413c63556f5c41b3c135fdcce0aa1adb177d9179

Demo payroll data is now better prepared for a Belgian multi-company setup, making it easier to create contracts and payslips with the needed accounting configuration already in place. Predictive bill tests were also adjusted to avoid naming conflicts when Belgian localization is installed, improving reliability without changing user workflows.