[IMP] point_of_sale,pos_restaurant: Improve demo data

addons/point_of_sale/data/point_of_sale_demo.xml

addons/point_of_sale/static/src/js/tests.js

addons/pos_restaurant/data/pos_restaurant_demo.xml

addons/pos_restaurant/static/img/th-burger.jpg

addons/pos_restaurant/static/img/th-cheeseburger.jpg

addons/pos_restaurant/static/img/th-club.jpg

addons/pos_restaurant/static/img/th-maki.jpg

addons/pos_restaurant/static/img/th-mozza.jpg

addons/pos_restaurant/static/img/th-pasta-4f.jpg

addons/pos_restaurant/static/img/th-pasta.jpg

addons/pos_restaurant/static/img/th-pizza.png

addons/pos_restaurant/static/img/th-salmon-avocado.jpg

addons/pos_restaurant/static/img/th-salmon.jpg

addons/pos_restaurant/static/img/th-sandwich.jpg

addons/pos_restaurant/static/img/th-temaki.jpg

addons/pos_restaurant/static/img/th-tuna.jpg