Add automated test for "Change Default Editor"

The manual test is here: https://test.xwiki.org/xwiki/bin/view/Administration%20Tests/Change%20Default%20Editor

The test verifies that the default wiki editor can be changed from Administration.

Matrix discussion: https://matrix.to/#/!ikPtGZaGWtyblizzlR:matrix.xwiki.com/$16215057674030WXYBa:matrix.xwiki.com?via=matrix.org&via=matrix.xwiki.com&via=dodsorf.as

Vincent Massol: Ilie Andriuta: checking. I don't think that https://github.com/xwiki/xwiki-platform/blob/73b07f8352d1716b9945b577d6e4a3334f058a77/xwiki-platform-core/xwiki-platform-administration/xwiki-platform-administration-test/xwiki-platform-administration-test-pageobjects/src/main/java/org/xwiki/administration/test/po/WYSIWYGEditorAdministrationSectionPage.java would help since the admin option is in Editing > Edit Mode and not in Editing > WYSIWYG
Vincent Massol: what you tried doing is a good strategy though: finding the PO for the UI and then finding usages of that PO in tests
Ilie Andriuta: yes, however, I couldn't find the tests for changing the editor
Vincent Massol: yes, when checking all classes extending AdministrationSectionPage there's none for "Editing > Edit Modes" that I can see which means we probably don't have tests for any of these settings
Ilie Andriuta: ok, I will create a JIRA task
thanks for looking into it!