selftests: nl_netdev: add a trivial Netlink netdev test

Add a trivial test using YNL.

  $ ./tools/testing/selftests/net/nl_netdev.py
  KTAP version 1
  1..2
  ok 1 nl_netdev.empty_check
  ok 2 nl_netdev.lo_check

Instantiate the family once, it takes longer than the test itself.

Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>