Add separate testing for unicast and multicast RAs.
Make the test more realistic by:
- Enabling forwarding. This ensures that the daemon actually
receives the RS.
- Adding a link-local route. This ensures that the daemon is
actually able to send a unicast response.
Bug: 154669942
Test: atest TetheringPrivilegedTests
Change-Id: Ibb1f51b5b1871657d7feff39335d3c71586cf64f