session-lockdown-no-deny: don't assert that all processes are running

The Tracker services are started on-demand, while Xorg might not be
installed.

Also continue testing if one of these assertions fails: just log it
as "not OK".

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/D3771