selftests: net: forwarding: change devlink resource support checking

As for the others, check help message output to find out if devlink
supports "resource" object.

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