[IMP] base,product,website_sale: move image fallback mixin -> variant

Only the variant is using this code, so it was a bad idea to make the mixin
more complicated than it had to be.

Update the image tests to make sure this is working correctly. The test were
only testing the size of the images, now they also test the actual content.

Part of task-1949729

closes odoo/odoo#32253

Signed-off-by: Jérémy Kersten (jke) <jke@openerp.com>