selftests: forwarding: Add IP-in-IP GRE flat topology test

Add a test that checks IP-in-IP GRE tunneling and MTU change of tunnel,
based on flat topology described in file ipip_lib.sh.

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>