route: introduce add interface route test cases

As a followup to D41330 and D41436, this patch introduces two new tests
for sbin/route: interface_route_v[46].

These tests fail without D41330.

Reviewed by: kp
Approved by: kp (mentor)
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")

(cherry picked from commit ca7edd0d2c339b8e6e5e52257ea6d4f22db02550)