selftests: forwarding: Add IP-in-IP GRE hierarchical topology with key test
Add a test that checks IP-in-IP GRE tunneling and MTU change of tunnel,
where a key is set. This test is based on hierarchical 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>