selftests: drv-net: factor out parsing of the env

The tests with a remote end will use a different class,
for clarity, but will also need to parse the env.
So factor parsing the env out to a function.

Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/20240420025237.3309296-3-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>