Add some tests for sbin/ndp binary

[lytboris_gmail.com]

Summary

This differential adds some tests for ndp binary including a test requested in https://reviews.freebsd.org/D49459

Test Plan

kyua test -k /usr/tests/sys/netinet/Kyuafile ndp

Expected output:

ndp:dad  ->  passed  [15.480s]
ndp:ndp_add_success  ->  passed  [0.042s]
ndp:ndp_del_success  ->  passed  [1.083s]
ndp:ndp_lookup_host  ->  passed  [0.088s]
ndp:pending_delete_if  ->  passed  [2.207s]
ndp:static_temporary  ->  passed  [0.123s]

Results file id is usr_tests_sys_netinet.20251023-133830-739277
Results saved to /home/..../.kyua/store/results.usr_tests_sys_netinet.20251023-133830-739277.db

6/6 passed (0 broken, 0 failed, 0 skipped)