selftests: drv-net: add require_XYZ() helpers for validating env

Wrap typical checks like whether given command used by the test
is available in helpers.

Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/20240420025237.3309296-8-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>