selftests: drv-net: validate the environment

Throw a slightly more helpful exception when env variables
are partially populated. Prior to this change we'd get
a dictionary key exception somewhere later on.

Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/20240425222341.309778-4-kuba@kernel.org


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