Add test coverage for \Drupal\filter\Plugin\Filter\FilterHtml::getHTMLRestrictions

Problem/Motivation

In 🐛 FilterHtml data loss when iframe and/or textarea is allowed Active we found that \Drupal\filter\Plugin\Filter\FilterHtml::getHTMLRestrictions broke when parsing certain elements such as iframe

This revealed there isn't explicit coverage for \Drupal\filter\Plugin\Filter\FilterHtml::getHTMLRestrictions which is used by other modules (media, ckeditor, fllter_format_audit) as a public API.

Steps to reproduce

Proposed resolution

Add explicit tests for \Drupal\filter\Plugin\Filter\FilterHtml::getHTMLRestrictions

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet