Add a Python test for the basic ops.

  # ./net/nl_netdev.py
  KTAP version 1
  1..3
  ok 1 nl_netdev.empty_check
  ok 2 nl_netdev.lo_check
  ok 3 nl_netdev.page_pool_check
  # Totals: pass:3 fail:0 xfail:0 xpass:0 skip:0 error:0
Reviewed-by: Petr Machata <petrm@nvidia.com>
Link: https://lore.kernel.org/r/20240412141436.828666-7-kuba@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>