Add unittest for DavCollectionsMultiFetchJob
Summary:
This also tests the XQuery and the expected XML response from the
server.
Test Plan: Passes (finally)
Reviewers: vkrause, mlaurent, dvratil, ervin
Reviewed By: vkrause
Subscribers: kde-pim
Tags: KDE PIM
Differential Revision: https://phabricator.kde.org/D28568