private String[] createBackupFilesWithVersion(Path backupDir, int startVersion, int endVersion, long baseModifiedTime,
      long interval, boolean isTemp) {
    String[] filenames = new String[endVersion - startVersion + 1];
    for (int i = startVersion; i <= endVersion; i++) {
      Stat stat = new Stat();
      stat.setVersion(i);
      stat.setMtime((baseModifiedTime + (i - startVersion) * interval) * 1000);
      String filename = BackupFileManager.getBackupFilename(stat.getVersion(), stat.getMtime() / 1000);
      if (isTemp) {
        filename = filename + BackupFileManager.SEP + BackupFileManager.TEMP_FILE_SUFFIX;
      }
      try {
        Files.createFile(backupDir.resolve(filename));
        filenames[i - startVersion] = filename;
      } catch (IOException e) {
        fail("Fail to create file " + filename);
      }
    }
    return filenames;
  }
Create backup files with version number without any content and return all the filenames.
@param backupDir The directory to create files under.
@param startVersion The starting version, inclusive.
@param endVersion The ending version, inclusive.
@param baseModifiedTime The starting version file's modified time.
@param interval The interval by which different backup files' modified time increases.
@param isTemp True if all the backup files should be temporary files.
@return Array of filenames.
ambry-account/src/test/java/com/github/ambry/account/BackupFileManagerTest.java