Fix test suite for systemctl preset.
There are multiple problems:
1/ first the behaviour of "dsh enable" is no longer the same when
the service was already installed or not... and as such the test suite
must be fixed in multiple places to ensure that the metadata
in /var/lib/systemd/ is properly purged before calling "enable" again
and expecting it to do its job.
2/ "systemctl preset" does collaborate with the running systemd to do its
job but the test suite uses its own filesystem namespace which is not
shared with the running systemd... and hence we get errors about
service files not existing in the systemd namespace even though they
exist in the testsuite namespace.
To work around this I modified the DEP-8 test to not use a separate
namespace... and marked the test as breaking the system so that it
does not get run on a real system. This has some consequences on some
other tests that assume that the /{lib,etc}/systemd/ directories are
empty. Those tests have been fixed along the way.
Updates that add new service files or remove existing ones are now also
tested (using update-state).
Furthermore, the Also= and Alias= statements in unit files are now
tested, too.
Raphaël Hertzog