nixos/tests/ferm: wait for DAD timeout before testing

The test has recently been failing due to the IPv6 address
on the server still being in the tentative state, when the
client sends its first request. The server will not start
using the IPv6 address until DAD has completed.

Scripted networking seems not to wait for DAD completion
before completing network-online.target, so let's switch
to networkd instead, which does.