private String[] createBackupFilesWithoutVersion(Path backupDir, int count, long baseModifiedTime, long interval,
      boolean isOld) {
    String[] filenames = new String[count];
    for (int i = 0; i < count; i++) {
      long modifiedTime = baseModifiedTime + i * interval;
      String timestamp = LocalDateTime.ofEpochSecond(modifiedTime, 0, BackupFileManager.zoneOffset)
          .format(BackupFileManager.TIMESTAMP_FORMATTER);
      String filename = timestamp + BackupFileManager.SEP + BackupFileManager.NEW_STATE_SUFFIX;
      if (isOld) {
        filename = timestamp + BackupFileManager.SEP + BackupFileManager.OLD_STATE_SUFFIX;
      }
      try {
        Files.createFile(backupDir.resolve(filename));
        filenames[i] = filename;
      } catch (IOException e) {
        fail("Fail to create file " + filename);
      }
    }
    return filenames;
  }
Create backup files in old format, without version number.
@param backupDir The directory to create files under.
@param count The number of backup files to create.
@param baseModifiedTime The first file's modified time.
@param interval The interval by which different backup files' modified time increases.
@param isOld True if all backup files should be suffix with .old.
@return Array of filenames.
ambry-account/src/test/java/com/github/ambry/account/BackupFileManagerTest.java