selftests: Add multipath tests for onlink flag

Add multipath tests for onlink flag: one test with onlink added to
both nexthops, then tests with onlink added to only 1 nexthop.

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
1 file changed