Generate IPv6 traffic in system tests

There is a problem with starting the Kea DHCPv6 daemon. It cannot bind the sockets. I didn't find any way to check if the binding would be available. The Kea has implemented a solution to retry the binding in kea#1716 (closed), but there is no possibility to check if the binding has already finished successfully. I opened kea#2434 (closed) to add an opportunity to inspect the binding status.

We can use the interface statuses to wait until the Kea is ready to allocate the leases.