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 :-)

1 files changed, 53 insertions, 0 deletions

diff --git a/test/TEST-13-NSPAWN-SMOKE/create-busybox-container b/test/TEST-13-NSPAWN-SMOKE/create-busybox-container
new file mode 100755
index 0000000000..868dfd852a
--- /dev/null
+++ b/test/TEST-13-NSPAWN-SMOKE/create-busybox-container
@@ -0,0 +1,53 @@
+#!/bin/bash
+
+set -e
+set -u
+set -o pipefail
+
+root="${1:?Usage $0 container-root}"
+mkdir -p "$root"
+mkdir "$root/bin"
+cp $(type -P busybox) "$root/bin"
+
+mkdir -p "$root/usr/lib"
+touch "$root/usr/lib/os-release"
+
+ln -s busybox "$root/bin/sh"
+ln -s busybox "$root/bin/cat"
+ln -s busybox "$root/bin/tr"
+ln -s busybox "$root/bin/ps"
+ln -s busybox "$root/bin/ip"
+
+mkdir -p "$root/sbin"
+cat <<'EOF' >"$root/sbin/init"
+#!/bin/sh
+
+printf "ps aufx:\n"
+ps aufx
+
+printf "/proc/1/cmdline:\n"
+printf "%s\n\n" "$(tr '\0' ' ' </proc/1/cmdline)"
+
+printf "/proc/1/environ:\n"
+printf "%s\n\n" "$(tr '\0' '\n' </proc/1/environ)"
+
+printf "/proc/1/mountinfo:\n"
+cat /proc/self/mountinfo
+printf "\n"
+
+printf "/proc/1/cgroup:\n"
+printf "%s\n\n" "$(cat /proc/1/cgroup)"
+
+printf "/proc/1/uid_map:\n"
+printf "%s\n\n" "$(cat /proc/1/uid_map)"
+
+printf "/proc/1/setgroups:\n"
+printf "%s\n\n" "$(cat /proc/1/setgroups)"
+
+printf "/proc/1/gid_map:\n"
+printf "%s\n\n" "$(cat /proc/1/gid_map)"
+
+printf "ip link:\n"
+ip link
+EOF
+chmod +x "$root/sbin/init"