Add a selftest for netdev generic netlink. For now there is only a
single test that exercises the `queue-get` API.

The test works with netdevsim by default or with a real device by
setting NETIF.

Add a timeout param to cmd() since ethtool -L can take a long time on
real devices.
Signed-off-by: David Wei <dw@davidwei.uk>
Link: https://lore.kernel.org/r/20240507163228.2066817-3-dw@davidwei.ukSigned-off-by: Jakub Kicinski <kuba@kernel.org>