nix command to run the unit tests

[cvengler]

Because the cgroup tests require a cargo test run in a systemd scope, I think we should offer a nix command to run this, such as systemd-cargo-test. Currently, we have to run the systemd-run --user --scope -- cargo test -- --nocapture

[wucke13]

This would benefit from something like #87 , so that we don't have to hardcode the partition image path to something inside target/.