Add automated test for "Login on Main Wiki with Local User From Subwiki"

The manual test is here: Login on Main Wiki with Local User From Subwiki

Existing automated coverage

None found.
Searched the wiki, authentication and administration test modules and XWikiAuthServiceImplTest (it only covers global users and wiki-prefixed usernames) for a subwiki-local user logging in on the main wiki.

Missing automated coverage

• Create a subwiki and a user local to it
• On the main wiki open the login form, submit the local user's username and password
• Assert the login is refused (error message displayed, still guest)

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

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