selftests: forwarding: Reorder (ar)ping arguments to obey POSIX getopt

selftests: forwarding: Reorder (ar)ping arguments to obey POSIX getopt

Quoted from musl wiki:

  GNU getopt permutes argv to pull options to the front, ahead of
  non-option arguments. musl and the POSIX standard getopt stop
  processing options at the first non-option argument with no
  permutation.

Thus these scripts stop working on musl since non-option arguments for
tools using getopt() (in this case, (ar)ping) do not always come last.
Fix it by reordering arguments.

Signed-off-by: David Yang <mmyangfl@gmail.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20250919053538.1106753-1-mmyangfl@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>