test: make the image bigger

I got some errors about lack of disk space... 100MB either way
shouldn't matter.
test: print the important commands to make debugging easier
units: systemd-logind fails hard without dbus

That is, without --enable-kdbus and kdbus running.

With --enable-kdbus things are more complicated, because dbus might be
necessary, if kdbus is missing at runtime. If it is not necessary,
the socket will be started, which is not imporant, but not the service.
test: add basic seccomp tests