test-nspawn-util: fix the test to actually find anything

test-nspawn-util: fix the test to actually find anything

We would use a relative path, looking for globs like
'lib/systemd/libsystemd-shared-*.so' under the build directory, and never find
anything.

The test was supposed to find library in the current installation. But we
cannot assume that the right library is installed, so the test only printed the
result for manual inspection. Thus nobody noticed when it broke. I think it
broke in c6134d3e2f1d1d17b32b6e06556cd0c5429bc78a, path-util: get rid of prefix_root().
But that commit doesn't compile because of changes in meson, so this is just
a guess.

Before:
/* test_systemd_installation_has_version */
Current installation has systemd >= 0: no
Current installation has systemd >= 231: no
Current installation has systemd >= 249: no
Current installation has systemd >= 999: no

With the fix:
$ build/test-nspawn-util
/* test_systemd_installation_has_version */
Found libsystemd shared at "/lib/systemd/libsystemd-shared-245.so.so", version 245 (OK).
Current installation has systemd >= 0: yes
Found libsystemd shared at "/lib/systemd/libsystemd-shared-245.so.so", version 245 (OK).
Current installation has systemd >= 231: yes
Found libsystemd shared at "/lib/systemd/libsystemd-shared-245.so.so", version 245 (too old).
Found libsystemd shared at "/lib/systemd/libsystemd-shared-251.so.so", version 251 (OK).
Current installation has systemd >= 251: yes
Found libsystemd shared at "/lib/systemd/libsystemd-shared-245.so.so", version 245 (too old).
Found libsystemd shared at "/lib/systemd/libsystemd-shared-251.so.so", version 251 (too old).
Found libsystemd shared at "/lib/systemd/libsystemd-shared-250.so.so", version 250 (too old).
Found libsystemd shared at "/usr/lib/systemd/libsystemd-shared-245.so.so", version 245 (too old).
Found libsystemd shared at "/usr/lib/systemd/libsystemd-shared-251.so.so", version 251 (too old).
Found libsystemd shared at "/usr/lib/systemd/libsystemd-shared-250.so.so", version 250 (too old).
Current installation has systemd >= 999: no

$ build/test-nspawn-util /var/lib/machines/rawhide
/* test_systemd_installation_has_version */
/* test_systemd_installation_has_version */
Found libsystemd shared at "/var/lib/machines/rawhide/lib/systemd/libsystemd-shared-251-rc1-1.fc37.so.so", version 251 (OK).
/var/lib/machines/rawhide has systemd >= 0: yes
Found libsystemd shared at "/var/lib/machines/rawhide/lib/systemd/libsystemd-shared-251-rc1-1.fc37.so.so", version 251 (OK).
/var/lib/machines/rawhide has systemd >= 231: yes
Found libsystemd shared at "/var/lib/machines/rawhide/lib/systemd/libsystemd-shared-251-rc1-1.fc37.so.so", version 251 (OK).
/var/lib/machines/rawhide has systemd >= 251: yes
Found libsystemd shared at "/var/lib/machines/rawhide/lib/systemd/libsystemd-shared-251-rc1-1.fc37.so.so", version 251 (too old).
Found libsystemd shared at "/var/lib/machines/rawhide/usr/lib/systemd/libsystemd-shared-251-rc1-1.fc37.so.so", version 251 (too old).
/var/lib/machines/rawhide has systemd >= 999: no

While at it, NULSTR_FOREACH → FOREACH_STRING.