Add tests for "add", "change" and "delete" functionality of /sbin/route.

Add tests to cover "add", "change" and "delete" functionality of /sbin/route
for ipv4 and ipv6. These tests for the existing route tool are the first step
towards creating libroute.

Submitted by: Ahsan Barkati
Sponsored by: Google, Inc. (GSoC 2020)
Reviewed by: kp, thj
Approved by: bz (mentor)
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D25220