netlink: Add tests when adding an interface route

Add tests for adding a route using an interface only (without an IP
address).

Reviewed by: rcm
Approved by: kp (mentor)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D41436

(cherry picked from commit 1db64f89363c97858961c4df0b7d02f3223723cf)