selftests: drv-net: reimplement the config parser

The shell lexer is not helping much, do very basic parsing
manually.

Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/20240425222341.309778-3-kuba@kernel.org


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