selftests: drv-net: add stdout to the command failed exception

ping prints all the info to stdout. To make debug easier capture
stdout in the Exception raised when command unexpectedly fails.

Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/20240416004556.1618804-2-kuba@kernel.org


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