Add automated test for "Join Open Wiki"

The manual test is here: Join Open Wiki

Existing automated coverage

• SubWikiIT#movePageToSubwiki: As a non-admin user on the Wiki Index, asserts the Join action link (WikiManager.JoinWiki?wikiId=subwiki) is offered for the open subwiki.
• WikiUserManagerScriptServiceTest#join: Unit test (mocks) of the join script API only.

Missing automated coverage

• As a non-member user on the Wiki Index click Join for an open subwiki and confirm with Yes
• Assert the confirmation message is displayed
• Assert the user is now a member of the subwiki and the Join action is no longer offered

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).