netinet6 tests: Add a regression test for default router handling

This serves as a regression test for commit a48df53e4249.

Reviewed by: bz, allanjude
MFC after: 2 weeks
Sponsored by: Klara, Inc.
Sponsored by: Bell Tower Integration
Differential Revision: https://reviews.freebsd.org/D46136

(cherry picked from commit feda329622bc77ab64ae5f0bf852743f4a037616)