selftests: fib_tests: Add test cases for netdev down

Check that IPv4 and IPv6 react the same when a netdev is being put
administratively down.

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