Improve debugging output on routing tests failure.

Most of the routing tests create per-test VNET, making
it harder to repeat the failure with CLI tools.
Provide an additional route/nexthop data on failure.

Differential Revision: https://reviews.freebsd.org/D29957
Reviewed by: kp
MFC after: 2 weeks

(cherry picked from commit bddae5c8a64dc6b292198945cbe676bb2158d438)