TestFormBlock.php

<?php

namespace Drupal\block_test\Plugin\Block;

use Drupal\Core\Block\BlockBase;

/**
 * Provides a block to test caching.
 *
 * @Block(
 *   id = "test_form_in_block",
 *   admin_label = @Translation("Test form block caching")
 * )
 */
class TestFormBlock extends BlockBase {
    
    /**
     * {@inheritdoc}
     */
    public function build() {
        return \Drupal::formBuilder()->getForm('Drupal\\block_test\\Form\\TestForm');
    }

}