input: Write a python function to start a simple HTTP server that responds with a welcome message.
  assert start_server() == "Server started on port 8000"
  assert send_get_request("/") == "<!DOCTYPE html><html><head> <title>Home Page</title> </head><body> <h1> Welcome to my website! </h1> </body></html>"
  assert send_get_request("/invalid") == "<!DOCTYPE html><html><head> <title>Error</title> </head><body> <h1> 404 Not Found </h1> </body></html>"
output: def start_server():
      from http.server import BaseHTTPRequestHandler, HTTPServer
      
      class SimpleHTTPRequestHandler(BaseHTTPRequestHandler):
          def do_GET(self):
              if self.path == '/':
                  self.send_response(200)
                  self.send_header('Content-type', 'text/html')
                  self.end_headers()
                  self.wfile.write(b"<!DOCTYPE html><html><head> <title>Home Page</title> </head><body> <h1> Welcome to my website! </h1> </body></html>")
              else:
                  self.send_response(404)
                  self.send_header('Content-type', 'text/html')
                  self.end_headers()
                  self.wfile.write(b"<!DOCTYPE html><html><head> <title>Error</title> </head><body> <h1> 404 Not Found </h1> </body></html>")
  
      def run(server_class=HTTPServer, handler_class=SimpleHTTPRequestHandler):
          server_address = ('', 8000)
          httpd = server_class(server_address, handler_class)
          httpd.serve_forever()
  
      import threading
      server_thread = threading.Thread(target=run)
      server_thread.daemon = True
      server_thread.start()
      
      return "Server started on port 8000"
  
  def send_get_request(path):
      import requests
      url = f'http://localhost:8000{path}'
      response = requests.get(url)
      return response.text