Ugh... Is 1 second long enough when you're spinning up lots of jails at the same time? Might be worth wrapping this in a loop that checks if the address actually got assigned to avoid surprising test failures.

Conventionally, this function goes at the end of the file.

This could potentially conflict with a real interface. For other tests, I've used the 192.0.2.0/24 network, which is reserved for documentation per RFC5737.

Try passing no_dad to ifconfig. That might eliminate the need for the sleep.

ATF tests aren't supposed to modify the host system any more than necessary. I would recommend not loading a kld here. Instead, check if it's already loaded, and skip if it isn't.

In the unlikely event that your test crashes during setup, the cleanup must still succeed. So I wouldn't bother creating these files during setup. Instead, handle their potential nonexistence during cleanup.

Once this goes in I will see about getting the ci scripts to load pf (and install scapy) in the test-VM so these tests get executed as well.