Add automated test for "Migrate XWiki using Distribution Wizard"

The manual test is here: Migrate XWiki using Distribution Wizard

Existing automated coverage

• UpgradeTest#upgrade: Starts an old data set on the new version, runs the Distribution Wizard (welcome, flavor, orphaned dependencies, extensions, report) and checks home page and extensions
• Upgrade84FileStoreTest#postUpdateValidate: Migrated attachments (one with spaces in its name) and a deleted attachment revision readable on main wiki and subwiki wiki1
• Upgrade1210CustomTest#postUpdateValidate: Likes preserved and a notification count for user1 after upgrade
• Upgrade1610Test#postUpdateValidate: Solr index migration

Missing automated coverage

• In an upgrade data set with a subwiki: assert nested and terminal pages with content and comments exist after migration on main wiki and subwiki.
• Assert untyped cross-wiki links (to subwiki:PageNS.WebHome from the main wiki, to xwiki:PageN.PageT from the subwiki) still point to existing pages.
• Assert Page Index > Deleted Pages and Deleted Attachments list the deleted entries on both wikis and that they can be opened.
• Assert the attachments LiveData pagination (next/previous) works on a migrated page.
• Assert an installed application (e.g. an AWM app) still lists its entries on both wikis.
• Assert the User Index shows Admin and global users on the main wiki, and local plus member global users on the subwiki.
• Assert a user's notifications (events and read status) and notification filter preferences are preserved.
• Assert something after "Check for updates" in UpgradeTest#extensionsStep (currently a TODO).
• Large (200 MB) and Office attachments can stay manual (impractical in CI).

Suggested location: xwiki-platform-distribution/xwiki-platform-distribution-flavor/xwiki-platform-distribution-flavor-test/xwiki-platform-distribution-flavor-test-upgrade (a version-specific upgrade test overriding postUpdateValidate(), plus its data set)

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