nixos/tests/systemd-networkd: fix eval
nixos/tests/systemd-networkd: fix eval

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] https://github.com/systemd/systemd/commit/1c30b174edc001537350036245ea632cb4e91cbb
[2] e9d13d37515cde47ec24410ca19866e68e5a7bd5

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