Add automated test for "Export and Import Partial Wiki"

The manual test is here: https://test.xwiki.org/xwiki/bin/view/Administration%20Tests/Export%20and%20Import%20Partial%20Wiki

The test verifies that some documents are properly exported and imported into another wiki from Administration.

Matrix discussion: https://matrix.to/#/!ikPtGZaGWtyblizzlR:matrix.xwiki.com/$16208068421650YGFUo:matrix.xwiki.com?via=matrix.org&via=matrix.xwiki.com&via=dodsorf.as

Simon Urli: I don't recall tests about that
Ilie Andriuta: I will create a task for automate it, and if an automated test is found for it afterwards, it can be closed
thanks, Simon Urli!