tests: Report D-Bus error when xfconfd is not auto-started

Description

This might make it easier to debug intermittent distcheck failures on CI:

• https://gitlab.xfce.org/xfce/xfconf/-/jobs/57708
• https://gitlab.xfce.org/xfce/xfconf/-/jobs/56604

** CRITICAL: xfconfd is not running and can not be autostarted
FAIL t-set-string (exit status: 1)

Testing

• make check passes locally