Make ExternalSSTFileTest::CompactionDeadlock more deterministic
Make ExternalSSTFileTest::CompactionDeadlock more deterministic

Summary:
It's not always true that `ASSERT_EQ(running_threads.load(), 2);`
Closes https://github.com/facebook/rocksdb/pull/1736

Differential Revision: D4374091

Pulled By: IslamAbdelRahman

fbshipit-source-id: 4f70bbd

SunguckLee/RocksDB,SunguckLee/RocksDB,bbiao/rocksdb,wat-ze-hex/rocksdb,vmx/rocksdb,SunguckLee/RocksDB,wat-ze-hex/rocksdb,Andymic/rocksdb,wat-ze-hex/rocksdb,vmx/rocksdb,bbiao/rocksdb,Andymic/rocksdb,Andymic/rocksdb,bbiao/rocksdb,wat-ze-hex/rocksdb,wat-ze-hex/rocksdb,bbiao/rocksdb,bbiao/rocksdb,vmx/rocksdb,wat-ze-hex/rocksdb,bbiao/rocksdb,bbiao/rocksdb,wat-ze-hex/rocksdb,SunguckLee/RocksDB,bbiao/rocksdb,SunguckLee/RocksDB,vmx/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,vmx/rocksdb,Andymic/rocksdb,vmx/rocksdb,Andymic/rocksdb,wat-ze-hex/rocksdb,SunguckLee/RocksDB,vmx/rocksdb,vmx/rocksdb