selftests: net: Add test cases for nexthop groups with invalid neighbors

Add test cases for hash threshold (multipath) nexthop groups with invalid
neighbors. Check that a nexthop with invalid neighbor is not selected when
there is another nexthop with a valid neighbor. Check that there is no
crash when there is no nexthop with a valid neighbor.

The first test fails before the previous commit in this series.

Signed-off-by: Benjamin Poirier <bpoirier@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20230719-nh_select-v2-4-04383e89f868@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>