Test running ping through two tinc daemons.

Test running ping through two tinc daemons.

This is a more complicated test with one tinc daemon using a tap interface
(therefore requiring root), and a second one using a multicast interface. A
separate program "pong" is listening on the same multicast address, and waits
for ARP and ICMP packets, responding to ICMP echo packets with replies.
This test doesn't require any configuration of the tap interface.