selftests: forwarding: Add devlink_lib.sh

This helper library contains wrappers to devlink functionality agnostic
to the underlying device.

Signed-off-by: Yuval Mintz <yuvalm@mellanox.com>
[petrm@mellanox.com: Split this out from another patch.]
Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>