TestFormBlock.php

<?php

declare (strict_types=1);
namespace Drupal\block_test\Plugin\Block;

use Drupal\Core\Block\Attribute\Block;
use Drupal\Core\Block\BlockBase;
use Drupal\Core\StringTranslation\TranslatableMarkup;

/**
 * Provides a block to test caching.
 */
class TestFormBlock extends BlockBase {
  
  /**
   * {@inheritdoc}
   */
  public function build() {
    return \Drupal::formBuilder()->getForm('Drupal\\block_test\\Form\\TestForm');
  }

}