support/tests: enhance the runtime systemd tests

Recent systemd bump has broken DBus dameon and DBus applications can no
longer find the daemon. So we want to catch those kind of failures
early.

We also want to check that the system as a whole is stable: no unit
should be failed.

Finally, ensure that we can read the jounrnal, even when we are doing our
tricks on read-only systems.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>