input: Write a function to start a simple HTTP server using http.server module.
  assert start_server() == "Server started at port 80"
  assert start_server(8080) == "Server started at port 8080"
  assert start_server(8000) == "Server started at port 8000"
output: def start_server(port=80):
      from http.server import HTTPServer, SimpleHTTPRequestHandler
      import threading
  
      def run_server():
          httpd = HTTPServer(('localhost', port), SimpleHTTPRequestHandler)
          httpd.serve_forever()
  
      server_thread = threading.Thread(target=run_server)
      server_thread.daemon = True
      server_thread.start()
      
      return f"Server started at port {port}"