selftests: forwarding: Add a test for basic IPv4 and IPv6 routing

Configure two hosts which are directly connected to the same router and
test IPv4 and IPv6 ping. Use a large MTU and check that ping is
unaffected.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>