Add automated test for "User and Group picker"

The manual test is here: User and Group picker.

Matrix discussion: https://matrix.to/#/!ikPtGZaGWtyblizzlR:matrix.xwiki.com/$1743063466130cmuEU:matrix.xwiki.com?via=matrix.org&via=matrix.xwiki.com&via=mtrx.nz

Simon Urli: Ilie Andriuta: Hi, I think you're looking at the wrong place. There's https://github.com/xwiki/xwiki-platform/blob/master/xwiki-platform-core/xwiki-platform-appwithinminutes/xwiki-platform-appwithinminutes-test/xwiki-platform-appwithinminutes-test-docker/src/test/it/org/xwiki/appwithinminutes/test/ui/UserClassFieldIT.java for testing the user picker in AWM
now I don't see a test in there for groups
there's also https://github.com/xwiki/xwiki-platform/blob/master/xwiki-platform-core/xwiki-platform-administration/xwiki-platform-administration-test/xwiki-platform-administration-test-docker/src/test/it/org/xwiki/administration/test/ui/UsersGroupsRightsManagementIT.java that might use the group picker but not in AWM
so I'd say we probably are missing an automated test for group picker in AWM
also https://test.xwiki.org/xwiki/bin/view/Administration%20Tests/User%20group%20displayer is about testing in a subwiki, while the existing test for user picker test in main wiki only
Ilie Andriuta: Thanks Simon Urli, I think I will create a task for this exact case to have it.