apparmor-session-lockdown: allow dpkg-query -W apertis-tests to fail

This test is run from the source tree, so there's no guarantee
that this query will return anything.

Reviewed-by: Philip Withnall <philip.withnall@collabora.co.uk>
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D3487