[IMP] website: test the images wall snippet

This commit separates the test that was added together with the "not
move images twice within image wall" fix because it requires additional
fixes for working in 16.0.

task-2990053

closes odoo/odoo#124608

X-original-commit: fa233f80
Signed-off-by: Arthur Detroux (ard) <ard@odoo.com>