input: Write a function to provide a command-line interface to download subtitles for movies and TV shows. The function should accept command-line arguments for the operations "download", "exist", "test", "list", "help", and "version". It should also allow specifying the language and subtitle site to use for downloading subtitles.
  
  ### Function Signature:
  def subtitles_client():
  
  ### Test Cases:
  
  1. **Test Case 1:**
     **Input:** 
     
     download /path/to/movie1 --lang=en --engine=opensubtitles
     
     **Output:** Downloads the English subtitle for `movie1` from the OpenSubtitles site.
  
  2. **Test Case 2:**
     **Input:** 
     
     exist /path/to/movie2 --lang=fr
     
     **Output:** Checks if the French subtitle for `movie2` exists on the default subtitle site.
  
  3. **Test Case 3:**
     **Input:** 
     
     test
     
     **Output:** Runs tests on all available subtitle engines.
  
  4. **Test Case 4:**
     **Input:** 
     
     list
     
     **Output:** Lists all available subtitle engine names.
  
  5. **Test Case 5:**
     **Input:** 
     
     help
     
     **Output:** Displays the help message for the command-line interface.
output: def subtitles_client():
      import sys
      import argparse
  
      parser = argparse.ArgumentParser(description="Download subtitles for movies and TV shows.")
      subparsers = parser.add_subparsers(dest="command")
  
      download_parser = subparsers.add_parser("download", help="Download subtitles for a movie or TV show.")
      download_parser.add_argument("path", help="Path to the movie or TV show file.")
      download_parser.add_argument("--lang", default="en", help="Language of the subtitles.")
      download_parser.add_argument("--engine", default="opensubtitles", help="Subtitle site to use.")
  
      exist_parser = subparsers.add_parser("exist", help="Check if subtitles exist for a movie or TV show.")
      exist_parser.add_argument("path", help="Path to the movie or TV show file.")
      exist_parser.add_argument("--lang", default="en", help="Language of the subtitles.")
  
      subparsers.add_parser("test", help="Run tests on all available subtitle engines.")
      subparsers.add_parser("list", help="List all available subtitle engine names.")
      subparsers.add_parser("help", help="Display the help message for the command-line interface.")
      subparsers.add_parser("version", help="Display the version of the command-line interface.")
  
      args = parser.parse_args()
  
      if args.command == "download":
          # Code to download subtitles
          pass
      elif args.command == "exist":
          # Code to check if subtitles exist
          pass
      elif args.command == "test":
          # Code to run tests
          pass
      elif args.command == "list":
          # Code to list available engines
          pass
      elif args.command == "help":
          parser.print_help()
      elif args.command == "version":
          print("Subtitles Client Version 1.0")