add rudimentary test harness, turn example-configuration into a test

tests/smoke/run.sh

@@ -0,0 +1,22 @@
+set -e
+NIXPKGS_ALLOW_UNSUPPORTED_SYSTEM=1 nix-build  -I liminix-config=./tests/smoke/configuration.nix -o smoke.img
+
+TESTS=$(cat <<"EOF"
+
+trap 'echo "command $(eval echo $BASH_COMMAND) failed with exit code $?"; exit $?' ERR
+dest_path=${TMPDIR}/smoke.img-$$
+echo $dest_path
+unsquashfs -q -d $dest_path smoke.img
+cd $dest_path;
+db=*-s6-rc-db/compiled/
+test -d $db
+chmod -R +w $db
+# check we have closure of config.services (lo.link.service exists only
+# as a dependency)
+test "$(s6-rc-db -c $db type lo.link.service)" = "oneshot"
+echo OK
+EOF
+     )
+
+
+nix-shell -p s6-rc -p squashfsTools --run "$TESTS" || exit 1