@Override
      public void onFileCopySuccess() {
        successFailCount.put("success", successFailCount.get("success") + 1);
      }
Tests when fileCopyHandler is failed and onFileCopyFailure is called.
and onFileCopySuccess is not called
ambry-file-transfer/src/test/java/com/github/ambry/filetransfer/FileCopyThreadTest.java