apparmor-libreoffice: emit stderr from unoconv

Tests that suppress debug output are user-hostile, and we should
stop doing that.

Reviewed-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D3636