Add automated test for "Migrate XWiki using Distribution Wizard"

Deferred for now: automating this test needs a new custom upgrade data set, so it stays manual.

The upgrade tests start the new XWiki version on an old data set, so the content the test checks after the migration has to exist in that data set beforehand. None of the existing data sets has it:

• Upgrade1710Test uses the stock xwiki-platform-distribution-flavor-data-hsqldb 17.10.0, a fresh flavor install: no subwiki and no user content.
• xwiki-enterprise-data-hsqldb 8.4.6 custom-filestore-6 (Upgrade84FileStoreTest) has a wiki1 subwiki, but it is a fixture for attachment storage: no comments, cross-wiki links, deleted attachments, application or local users.
• xwiki-platform-distribution-flavor-data-hsqldb 12.10.11 custom-0 (Upgrade1210CustomTest) only holds likes and notifications.

What it would take:

1. Run a 17.10.x jetty-hsqldb distribution and create the content: a subwiki with local users, nested and terminal pages with content and comments on both wikis, untyped cross-wiki links in both directions, deleted pages and attachments on both wikis, an AWM application with entries on both wikis, and notifications (read and unread) plus notification filter preferences for a user.
2. Zip the data directory and publish it to Nexus as xwiki-platform-distribution-flavor-data-hsqldb:17.10.x:custom-N.
3. Add an xwiki-platform-distribution-flavor-test-upgrade-1710-custom module whose Upgrade1710CustomTest#postUpdateValidate checks the migrated content, the Deleted Pages / Deleted Attachments of the Page Index, the attachments LiveData pagination, the application entries, the User Index on both wikis and the notification state.

Independently of the data set, UpgradeTest#extensionsStep still has a TODO to check something after "Check for updates". The large (200 MB) and Office attachment steps should stay manual in any case.