input: Write a function to start a simple HTTP server on a specified port and return the port number.
  assert start_http_server(8080) == 8080
  assert start_http_server(9090) == 9090
  assert start_http_server(8000) == 8000
output: def start_http_server(port):
      from http.server import HTTPServer, SimpleHTTPRequestHandler
      import threading
  
      def run_server():
          httpd = HTTPServer(('localhost', port), SimpleHTTPRequestHandler)
          httpd.serve_forever()
  
      thread = threading.Thread(target=run_server)
      thread.daemon = True
      thread.start()
      
      return port