selftests: net-drv: stats: sanity check netlink dumps

Sanity check netlink dumps, to make sure dumps don't have
repeated entries or gaps in IDs.

Reviewed-by: Petr Machata <petrm@nvidia.com>
Link: https://patch.msgid.link/20241213152244.3080955-6-kuba@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>