ndp tests: Fix flakiness in ndp_slaac_default_route

The test sends RAs in order to test SLAAC handling in another host.
The router needs to also set net.inet6.ip6.forwarding=1, otherwise NAs
sent from it have the ROUTER flag clear, and upon receiving such an NA
the host will automatically delete routes learned from the router.

Fixes: feda329622bc ("netinet6 tests: Add a regression test for default router handling")
MFC after: 1 week
Sponsored by: Klara, Inc.

(cherry picked from commit 1eb727727a9acb5f1e66e3f70b0146e7c9c5f710)