Fix TestXdgShellClient::testDesktopFileName
Summary:
Allows AbstractClient::m_desktopFileName be an absolute path, which is
used by tests.
Test Plan: Test passes, other things still work, including telegram.
Reviewers: KWin, zzag
Reviewed By: KWin, zzag
Subscribers: kwin
Tags: KWin
Differential Revision: https://phabricator.kde.org/D29758