[IMP] base,web,website_sale: add tests for product image and mixin

This commit will add tests for the PR #30656 which added the following commit:
7daa85dd

The tests in the current commit achieve 100% code coverage for:
- the `image.mixin`
- the overridden `image.mixin` methods in `product.product`
- the `_get_images` methods of ecommerce
- the size logic in method `content_image` (route `/web/image`)

PR: #31181