Exit value 1 instead of a random non-zero value.
Fix multicast device.
Add two more test scripts.
Don't return zero-length packets when receiving multicast loopback packets.
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.