Add tests for remove_cache_setting_from_syndicate_blocks post-update hook

Problem/Motivation

In 🐛 The Sitemap blocks' settings have no config schema Needs work , we discovered that the cache, and its sub-key max_age configuration options in SitemapSyndicateBlock, and we wrote sitemap_post_update_remove_cache_setting_from_syndicate_blocks() to remove the cache key from block configuration.

But because 3545794 was blocking Drupal CMS, we decided to defer writing tests for that change to this ticket.

Proposed resolution

Write fixtures and tests for sitemap.post_update.php's sitemap_post_update_remove_cache_setting_from_syndicate_blocks().

Remaining tasks

1. Write a merge request
2. Review and feedback
3. RTBC and feedback (likely skip-able if it's just tests)
4. Commit
5. Release

User interface changes

None.

API changes

None.

Data model changes

None.