selftests: forwarding: Test IPv4 routes with IPv6 link-local nexthops

In addition to IPv4 multipath tests with IPv4 nexthops, also test IPv4
multipath with nexthops that use IPv6 link-local addresses.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>