CPT Class as Plugin

.php

<?php
/**
 * Plugin.
 *
 * Plugin Name:         Plugin
 * Version:             1.0.0
 * License:             MIT
 * Text Domain:         plugin
 */
class FlushTester
{
 public function __construct() {
  $this->register();
 }

 public function custom_post_type() {
  register_post_type(
   'test',
   [
    'public'      => true,
    'has_archive' => true,
    'rewrite'     => ['slug' => 'testing'],
    'label'       => esc_html__( 'Test', 'flushtester' ),
    'supports'    => ['title', 'editor', 'thumbnail'],
    'menu_icon'   => 'dashicons-screenoptions',
   ]
  );
 }

 public function register() {
  add_action( 'init', [ $this, 'custom_post_type' ] );
 }

 static function flush() {
  delete_option( 'rewrite_rules' );
  // flush_rewrite_rules();
 }
}

$dpc = new FlushTester();

register_activation_hook( __FILE__, [ FlushTester::class, 'flush' ] );
register_deactivation_hook( __FILE__, [ FlushTester::class, 'flush' ] );