tests/netinet6: Add SLAAC and RA validation tests to ndp

[pouria]

Summary

• RA hop limit validation
• RA source address validation
• Multi router RA validation
• Two hour rule RA validation
• SLAAC onlink prefix switching test

Test Plan

# kyua test -k /usr/tests/Kyuafile sys/netinet6/ndp
sys/netinet6/ndp:ndp_prefix_lifetime_extend  ->  passed  [0.738s]
sys/netinet6/ndp:ndp_add_gu_success  ->  passed  [2.801s]
sys/netinet6/ndp:ndp_prefix_len_mismatch  ->  passed  [3.009s]
sys/netinet6/ndp:ndp_del_gu_success  ->  passed  [3.301s]
sys/netinet6/ndp:ndp_grand_linklayer_event  ->  passed  [5.151s]
sys/netinet6/ndp:ndp_slaac_default_route  ->  passed  [6.204s]
sys/netinet6/ndp:ndp_slaac_twohour_rule  ->  passed  [6.474s]
sys/netinet6/ndp:ndp_input_validation_src_linklocal  ->  passed  [6.798s]
sys/netinet6/ndp:ndp_multirouter_pref  ->  passed  [6.247s]
sys/netinet6/ndp:ndp_input_validation_hlim  ->  passed  [5.869s]
sys/netinet6/ndp:ndp_slaac_switch_onlink_prefix  ->  passed  [10.435s]
sys/netinet6/ndp:ndp_prefix_lifetime  ->  passed  [17.059s]

Results file id is usr_tests.20260328-193320-696702
Results saved to /root/.kyua/store/results.usr_tests.20260328-193320-696702.db

12/12 passed (0 broken, 0 failed, 0 skipped)

[pouria]

tests/sys/netinet6/ndp.sh

580–582

XXX comment is for D55449 and ideally a node with multiple routers on its link should have multiple default routes with different nexthop weights.

664–673

FYI, this behavior seemed odd.
we update the prefix valid lifetime in ndp -p when we shouldn't.
However, It's not going to be a problem since we don't update the actual vltime on ifa.