Add automated test for "Retrieve Tag"

The manual test is here: Retrieve Tag

Existing automated coverage

• TagIT#addRemoveTag: Step 1: adding tags to a page through the UI
• TagIT#getAllTags: Tags of a page are returned by $xwiki.tag.getAllTags() (script API, not Main.Tags)
• TagCloudPageTest#withTags: Unit level: the tag cloud lists all tags with links to Main.Tags?do=viewTag, from mocked query results

Missing automated coverage

• Add distinct tags to a nested page (X.WebHome) and a terminal page (X.Y)
• Go to Main.Tags and assert the tag cloud contains all those tags
• Optionally click each tag and assert the viewTag page lists the right page

Suggested location: xwiki-platform-core/xwiki-platform-tag/xwiki-platform-tag-test/xwiki-platform-tag-test-docker/src/test/it/org/xwiki/tag/test/ui/TagIT.java

Found by comparing the XWiki 18.4.4 manual test runs against the automated tests on master (a0055f10a1a).