tests/ignition: check file ownership for system users

This ensure that entries in Ignition configuration can reference
system users even if not present in ostree commit (e.g. `zincati).

(cherry picked from commit dd25144)

Author: lucab

tests/kola/ignition/sysusers/config.fcc

@@ -0,0 +1,12 @@
+---
+variant: fcos
+version: 1.0.0
+storage:
+  files:
+    - path: /etc/zincati/config.d/00-dummy-placeholder.toml
+      mode: 0644
+      user:
+        name: "zincati"
+      contents:
+        inline: |
+          # Dummy placeholder

tests/kola/ignition/sysusers/test.sh

@@ -0,0 +1,20 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+ok() {
+    echo "ok" "$@"
+}
+
+fatal() {
+    echo "$@" >&2
+    exit 1
+}
+
+TARGET="/etc/zincati/config.d/00-dummy-placeholder.toml"
+OWNER=$(stat -c '%U' "${TARGET}")
+
+# make sure the placeholder file is owned by the proper system user.
+if test "${OWNER}" != 'zincati' ; then
+    fatal "unexpected owner of ${TARGET}: ${OWNER}"
+fi
+ok "placeholder file correctly owned by zincati user"