tests: add smoke test for systemd-nspawn

tests: add smoke test for systemd-nspawn

Basically, this test runs:
```
    systemd-nspawn --register=no -D "$_root" -b
    systemd-nspawn --register=no -D "$_root" --private-network -b
    systemd-nspawn --register=no -D "$_root" -U -b
    systemd-nspawn --register=no -D "$_root" --private-network -U -b
```
and exports the `UNIFIED_CGROUP_HIERARCHY=[yes|no]`, `SYSTEMD_NSPAWN_USE_CGNS=[yes|no]`

Inspired by
* systemd#3589 (comment)
* systemd#4372 (comment)
* systemd#4223 (comment)
* systemd#1555

and so on :-)