Summary:
current `clean` target in Makefile does not remove parallel test
binaries. Fix this.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5629

Test Plan:
(on devserver)
Take file_reader_writer_test for instance.
```
$make -j32 file_reader_writer_test
$make clean
```
Verify that binary file 'file_reader_writer_test' is delete by `make clean`.

Differential Revision: D16513176

Pulled By: riversand963

fbshipit-source-id: 70acb9f56c928a494964121b86aacc0090f31ff6