input: Write a function to start a simple HTTP server on a specified port.
  assert start_http_server(8080)=="Server running at http://localhost:8080/"
  assert start_http_server(9090)=="Server running at http://localhost:9090/"
  assert start_http_server(8000)=="Server running at http://localhost:8000/"
output: def start_http_server(port):
      import http.server
      import socketserver
      import threading
  
      handler = http.server.SimpleHTTPRequestHandler
      httpd = socketserver.TCPServer(("", port), handler)
  
      def server_thread():
          httpd.serve_forever()
  
      thread = threading.Thread(target=server_thread)
      thread.daemon = True
      thread.start()
  
      return f"Server running at http://localhost:{port}/"