selftests: forwarding: Add IP tunneling lib

Add a library with helper functions, to be used in testing IP-in-IP and GRE
tunnels, both in flat and in hierarchical topologies.
The topologies used in this library cover the three scenarios of tunnels -
a tunel with no bound device, a tunnel with bound device in the same VRF
and a tunnel with a bound device in a different VRF.

Signed-off-by: Nir Dotan <nird@mellanox.com>
Reviewed-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>