selftests: forwarding: devlink_lib: Allow tests to define devlink device

For tests that create their network interfaces dynamically or do not use
interfaces at all (as with netdevsim) it is useful to define their own
devlink device instead of deriving it from the first network interface.

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