Add an ATF test for IPv6 SLAAC with multiple fibs

Tests that an interface can get a SLAAC address and that it inserts its
routes into the correct fib. Does not test anything to do with NDP.

PR: 196361
Reviewed by: Erick Turnquist <jhujhiti@adjectivism.org>
MFC after: 3 weeks
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D9776