selftests: forwarding: lib: rewrite processing of command line arguments

selftests: forwarding: lib: rewrite processing of command line arguments

The piece of code which processes the command line arguments and
populates NETIFS based on them is really unobvious. Rewrite it so that
the intention is clear and the code is easy to follow.

Suggested-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Link: https://patch.msgid.link/20260407102058.867279-1-ioana.ciornei@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>