Add test for regression fixed in r329743

[rstone]

Summary

In r264986, the behaviour of route(8) regresssed and it became
no longer possible to modify a route via "route change" unless
you explicitly specified the route's gateway. Add an explicit
test for this behaviour to ensure that this doesn't happen again.

[asomers]

Most of the other tests in this file relate specifically to multifib functionality. I think your test should go into a new file. You can pull the helper routines out into a .subr file.

[rstone]

Move non-fib test into separate test file

[asomers]

Where is route.subr? And where did ipv4_move_subnet_route come from? I think you used the wrong base revision to generate the review.

[rstone]

In D14559#308712, @asomers wrote:

Where is route.subr? And where did ipv4_move_subnet_route come from? I think you used the wrong base revision to generate the review.

This based off of the change in https://reviews.freebsd.org/D14547

[asomers]

Why don't you just combine this review with that one? It would be easier to review that way.