Add automated test for "Extend User Profile"

The manual test is here: https://test.xwiki.org/xwiki/bin/view/Administration%20Tests/Extend%20User%20Profile

The test verifies that new fields can be added to an user's profile.

Matrix discussion: https://matrix.to/#/!ikPtGZaGWtyblizzlR:matrix.xwiki.com/$16203874748822GvlxZ:matrix.xwiki.com?via=matrix.org&via=matrix.xwiki.com&via=dodsorf.as

Vincent Massol: Ilie Andriuta: so this is about checking that the user can override the default editor to use right?
Ilie Andriuta: no
it's about adding fields to its profile
I mean properties to extend it
Vincent Massol: ah yes ok
Indeed I dont' recall any test for that, a pity
cc Enygma in case he remembers but looking at https://jira.xwiki.org/browse/XWIKI-7626 I don't think we added any test
Ilie Andriuta: ok, I'll open a task and will be closed then if a test is found afterwards if the case
Vincent Massol: yes ok, I'm pretty sure we don't have tests
Ilie Andriuta: thanks Vincent Massol for looking into it