In `systemd-243` the option `FwMark` in the `[WireGuard]` section of
a `.netdev`-unit has been renamed to `FirewallMark`[1]. Due to the
removal of deprecated options in our `networkd` module[2] the evaluation
of this test doesn't work.
Renaming the option to its new name fixes the issue.
[1] 1c30b174ed
[2] e9d13d3751