Test two tinc daemons using network namespaces.

Testing multiple daemons connecting to each other on the same computer is
usually difficult, because connections to local IP addresses will bypass most
of the network stack. However, recent versions of Linux support network
namespaces, which can isolate network interfaces. We use this to isolate the
virtual interface of the daemons from each other, so we get the behaviour as if
the daemons were each running on their own machine. This can also be used for
more complicated tests (including those with firewall rules) without disturbing
the real network setup of the host computer.