selftests: fib_nexthops: do not mark skipped tests as failed

selftests: fib_nexthops: do not mark skipped tests as failed

The current test marks all unexpected return values as failed and sets ret
to 1. If a test is skipped, the entire test also returns 1, incorrectly
indicating failure.

To fix this, add a skipped variable and set ret to 4 if it was previously
0. Otherwise, keep ret set to 1.

Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Link: https://patch.msgid.link/20250220085326.1512814-1-liuhangbin@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>