Add basic tests covering routing table and lltable manipulations via rtsock.
Having such tests help monitoring our ABI stability and eases testing upcoming routing changes.
As most of the tests involve operation on ARP/ND/route entires, test setup/teardown operations include
* creating new interface (currently `tap`, as it mimics ethernet pretty well)
* assigning v4/v6 address prefixes according to the configuration
* adding/deleting ARP/ND/route entries (all within specified v4/v6 prefixes)
* cleaning up by destroying the interface
Currently existing test variables:
* rtsock.ifname: interface to operate on, defaults to `tap4242`
* rtsock.create_interface: whether to auto-create/destroy the interface, default to `true`
* rtsock.v4prefix: IPv4 prefix to operate on (subprefixes can be created), defaults to `192.0.2.0/24` (ipv4 documentation prefix)
* rtsock.v6prefix: IPv6 prefix to operate on (subprefixes can be created), defaults to `2001:DB8::/32` (ipv6 documentation prefix)
* rtsock.remote_lladdr: mac address to use for arp/nd entries, defaults to `00-00-5E-00-53-FF` (IANA OUI documentation)