input: Write a python function to start a simple HTTP server that responds with "Hello, world!" on a GET request.
  assert start_server() == 'Server started on localhost:8000'
  assert send_get_request('http://localhost:8000') == 'Hello, world!'
  assert send_get_request('http://localhost:8000/test') == 'Hello, world!'
output: def start_server():
      from http.server import BaseHTTPRequestHandler, HTTPServer
      
      class SimpleHTTPRequestHandler(BaseHTTPRequestHandler):
          def do_GET(self):
              self.send_response(200)
              self.send_header('Content-type', 'text/plain')
              self.end_headers()
              self.wfile.write(b'Hello, world!')
      
      server_address = ('', 8000)
      httpd = HTTPServer(server_address, SimpleHTTPRequestHandler)
      
      from threading import Thread
      def run_server():
          httpd.serve_forever()
      
      server_thread = Thread(target=run_server)
      server_thread.setDaemon(True)
      server_thread.start()
      
      return 'Server started on localhost:8000'
  
  def send_get_request(url):
      import requests
      response = requests.get(url)
      return response.text