Add automated test for "Add And Delete Users To A Wiki"

The manual test is here: Add And Delete Users To A Wiki

Existing automated coverage

None found.
Searched all tests and page objects for WikiManager.WikiUsers, 'Current Members', 'Add or invite users to wiki' and their translation keys; only WikiUserManagerScriptServiceTest unit-tests the addMember/removeMember script API with mocks.

Missing automated coverage

• Create a subwiki and a global user on the main wiki
• On the subwiki go to Administer Wiki > Users & Rights > Users, pick the user in 'Add or invite users to wiki', click Add and assert it appears in Current Members
• Click Remove for that user and assert it's no longer listed in Current Members
• Add a page object for the WikiManager.WikiUsers administration section

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).