systemd-stable/test/test-execute/exec-supplementarygroups-multiple-groups-withuid.service

[Unit]
Description=Test for Supplementary Group with multiple groups and Uid=1

[Service]
ExecStart=/bin/sh -x -c 'test "$$(id -G)" = "1 2 3" && test "$$(id -g)" = "1" && test "$$(id -u)" = "1"'
Type=oneshot
User=1
SupplementaryGroups=1 2 3