test: fix networkd-test.py rate limiting and dynamic user

test: fix networkd-test.py rate limiting and dynamic user

 - Reset systemd-networkd.service before each test run, to avoid running
   into restart limits.
 - Our networkd-test-router.service unit needs to run as root and thus
   can't use `User=`; but networkd still insists on the
   `systemd-network` system user to exist, so create it.