input: Write a function to start an HTTP server on a specified port using the http.server module.
  assert start_http_server(8080)=="Serving HTTP on port 8080 ..."
  assert start_http_server(8000)=="Serving HTTP on port 8000 ..."
  assert start_http_server(3000)=="Serving HTTP on port 3000 ..."
output: def start_http_server(port):
      import http.server
      import socketserver
      from threading import Thread
  
      class Handler(http.server.SimpleHTTPRequestHandler):
          pass
  
      httpd = socketserver.TCPServer(("", port), Handler)
      
      def server_thread():
          httpd.serve_forever()
  
      thread = Thread(target=server_thread)
      thread.daemon = True
      thread.start()
  
      return f"Serving HTTP on port {port} ..."