test: rework how images are created

test: rework how images are created

Before, we'd create a separate image for each test, in
/var/tmp/systemd-test.XXXXX/rootdisk.img. Most of the images
where very similar, except that each one had some unit files installed
specifically for the test. The installation of those custom unit files
was removed in previous commits (all the unit files are always installed).

The new approach is to only create as few distinct images as possible.
We have:
default.img: the "normal" image suitable for almost all the tests
basic.img: the same as default image but doesn't mask any services
cryptsetup.img: p2 is used for encrypted /var
badid.img: /etc/machine-id is overwritten with stuff
selinux.img: with selinux added for fun and fun
and a few others:

ls -l build/test/*img
lrwxrwxrwx 1 root root 38 Mar 21 21:23 build/test/badid.img -> /var/tmp/systemd-test.PJFFeo/badid.img
lrwxrwxrwx 1 root root 38 Mar 21 21:17 build/test/basic.img -> /var/tmp/systemd-test.na0xOI/basic.img
lrwxrwxrwx 1 root root 43 Mar 21 21:18 build/test/cryptsetup.img -> /var/tmp/systemd-test.Tzjv06/cryptsetup.img
lrwxrwxrwx 1 root root 40 Mar 21 21:19 build/test/default.img -> /var/tmp/systemd-test.EscAsS/default.img
lrwxrwxrwx 1 root root 39 Mar 21 21:22 build/test/nspawn.img -> /var/tmp/systemd-test.HSebKo/nspawn.img
lrwxrwxrwx 1 root root 40 Mar 21 21:20 build/test/selinux.img -> /var/tmp/systemd-test.daBjbx/selinux.img
lrwxrwxrwx 1 root root 39 Mar 21 21:21 build/test/test08.img -> /var/tmp/systemd-test.OgnN8Z/test08.img

I considered trying to use the same image everywhere. It would probably be
possible, but it would be very brittle. By using separate images where it is
necessary we keep various orthogonal modifications independent.

The way that images are cached is complicated by the fact that we still
want to keep them in /var/tmp. Thus, an image is created on first use and
linked to from build/test/ so it can be found by other tests.

Tests cannot be run in parallel. I think that is an acceptable limitation.
Creation of the images was probably taking more resources then the actual
tests, so we should be better off anyway.