selftests: drv-net: add config for netdevsim

Real driver testing will obviously require enabling more
options, but will require more manual setup in the first
place. For CIs running purely software tests we need
to enable netdevsim.

Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/20240416004556.1618804-3-kuba@kernel.org


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