net-test: fix nduseropt parsing when multiple are present

Tested: manually on UML
  (on kernels both with and without PIO nduseropt fix)
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Bug: 363210101
(cherry picked from commit 79cb2927bbe0449d690b501df233149d576840bb)
Cherrypick-From: https://android-review.googlesource.com/q/commit:7f4de3c4c9502ea9e381aa66d96bca5c7fe8f384
Merged-In: Ia75574fad074914e60b227bc3ec788137175999e
Change-Id: Iafc98babfc451125e8de7700a773d36b15a67b29