selftests: drv-net: construct environment for running tests which require an endpoint

Nothing surprising here, hopefully. Wrap the variables from
the environment into a class or spawn a netdevsim based env
and pass it to the tests.

Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/20240420025237.3309296-4-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>