delete 2nd level children for default env
delete 2nd level children for default env

Summary: ensure no 2nd level children under test_dir_

Test Plan: env_basic_test on 4 envs

Reviewers: andrewkr

Reviewed By: andrewkr

Subscribers: andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D59979

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