mkosi: Add testuser and tar to system image

The integration tests are installed into the image
with the intention that it should be possible to run those tests,
but those tests require the named user testuser
and tar is needed for machined-import

mkosi.images/system/mkosi.conf

@@ -33,6 +33,7 @@ Packages=
         strace
         systemd
         tmux
+        tar
         tree
         udev
         util-linux

mkosi.images/system/mkosi.postinst.chroot

@@ -83,3 +83,12 @@ if [ "$ID" = "centos" ] && [ "$VERSION" = "8" ]; then
     alternatives --install /usr/bin/python3 python3 /usr/bin/python3.9 1
     alternatives --set python3 /usr/bin/python3.9
 fi
+
+mkdir -p /usr/lib/sysusers.d
+cat >/usr/lib/sysusers.d/testuser.conf <<EOF
+u	testuser	4711	"Test User"	/home/testuser
+EOF
+mkdir -p /usr/lib/tmpfiles.d
+cat >/usr/lib/tmpfiles.d/testuser.conf <<EOF
+q	/home/testuser	0700	4711	4711
+EOF