fix potential test SleepingTask race condition
fix potential test SleepingTask race condition

Summary: Make sure SleepingTask has bene run before it goes out of scope.

Test Plan: run test

Reviewers: kradhakrishnan

Reviewed By: kradhakrishnan

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D52581

OverlordQ/rocksdb,facebook/rocksdb,wat-ze-hex/rocksdb,ryneli/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB,vmx/rocksdb,hobinyoon/rocksdb,norton/rocksdb,tsheasha/rocksdb,Andymic/rocksdb,flabby/rocksdb,norton/rocksdb,norton/rocksdb,wenduo/rocksdb,jalexanderqed/rocksdb,tsheasha/rocksdb,bbiao/rocksdb,wenduo/rocksdb,facebook/rocksdb,bbiao/rocksdb,hobinyoon/rocksdb,vmx/rocksdb,tsheasha/rocksdb,ryneli/rocksdb,flabby/rocksdb,bbiao/rocksdb,ryneli/rocksdb,wenduo/rocksdb,flabby/rocksdb,vmx/rocksdb,norton/rocksdb,SunguckLee/RocksDB,wat-ze-hex/rocksdb,norton/rocksdb,wat-ze-hex/rocksdb,wat-ze-hex/rocksdb,OverlordQ/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,flabby/rocksdb,facebook/rocksdb,OverlordQ/rocksdb,tsheasha/rocksdb,OverlordQ/rocksdb,OverlordQ/rocksdb,wat-ze-hex/rocksdb,vmx/rocksdb,flabby/rocksdb,wenduo/rocksdb,norton/rocksdb,wenduo/rocksdb,norton/rocksdb,jalexanderqed/rocksdb,tsheasha/rocksdb,jalexanderqed/rocksdb,jalexanderqed/rocksdb,tsheasha/rocksdb,facebook/rocksdb,bbiao/rocksdb,flabby/rocksdb,Andymic/rocksdb,flabby/rocksdb,wat-ze-hex/rocksdb,OverlordQ/rocksdb,hobinyoon/rocksdb,Andymic/rocksdb,facebook/rocksdb,jalexanderqed/rocksdb,jalexanderqed/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,SunguckLee/RocksDB,wenduo/rocksdb,ryneli/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,wat-ze-hex/rocksdb,hobinyoon/rocksdb,wat-ze-hex/rocksdb,SunguckLee/RocksDB,wenduo/rocksdb,ryneli/rocksdb,vmx/rocksdb,hobinyoon/rocksdb,hobinyoon/rocksdb,vmx/rocksdb,tsheasha/rocksdb,wenduo/rocksdb,vmx/rocksdb,facebook/rocksdb,hobinyoon/rocksdb,ryneli/rocksdb,jalexanderqed/rocksdb,tsheasha/rocksdb,bbiao/rocksdb,Andymic/rocksdb,bbiao/rocksdb,vmx/rocksdb,norton/rocksdb,hobinyoon/rocksdb,OverlordQ/rocksdb,facebook/rocksdb,Andymic/rocksdb,ryneli/rocksdb,facebook/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb