User and group systemd-network are created from
sysusers.d/systemd-network.conf, which is only copied into the test
image when building with -Dnetworkd=true. This means that if
-Dnetworkd=false is used, the user and the group don't exist, which
causes the test to fail.
Use a locally created user and group to avoid that.
David Tardon
a11278ce63