add sched_yield into sleep-gettime-test.cpp
add sched_yield into sleep-gettime-test.cpp

hemingcui/xtern_with_paxos,columbia/xtern,columbia/xtern,columbia/xtern,columbia/xtern,hemingcui/xtern_with_paxos,columbia/xtern,columbia/xtern,hemingcui/xtern_with_paxos,hemingcui/xtern_with_paxos,hemingcui/xtern_with_paxos