knowledge_pad_ui_test: use an RSS from erp5.com for the test
Test should not depend on external websites, we already use erp5.com for some other tests, so let's use it also for RSS. /reviewed-on nexedi/erp5!1028
Test should not depend on external websites, we already use erp5.com for some other tests, so let's use it also for RSS. /reviewed-on nexedi/erp5!1028