libreoffice: work in a directory that LibreOffice can legitimately write

This means we don't have to worry about whether the ${TESTDIR}
is allowed by its AppArmor profile (it shouldn't really be).

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/D3632