Add XSS testing for RSS descriptions

Problem/Motivation

A follow-up for 🐛 RssResponseCdata filtering out common html tags Active - to add the test coverage in MR https://git.drupalcode.org/project/drupal/-/merge_requests/11023 to core to ensure that XSS is filtered or double escaped when it is in an RSS description field.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet