tests_: Add integration tests for systemd units and unconfined access

Add a few more integration tests which check that restarting various
parts of the daemon and its backends don’t cause the others to fail; and
that unconfined clients can’t access the SDK API.

Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Signed-off-by: Philip Withnall <philip.withnall@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D3168