selftests: net: add fdb nexthop tests

This commit adds ipv4 and ipv6 fdb nexthop api tests to fib_nexthops.sh.

Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>