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