Rendarable array to html string

Rendarable array to html string

    $entity = EntityTest::create();
    $entity->field_datasheet->target_id = $this->file->id();
    $entity->save();
    $output = $this->renderTestEntity($entity->id());
    $expected_link = Link::fromTextAndUrl($this->field_label, Url::fromUri(file_create_url($this->file->getFileUri())));
    $expected_link = $expected_link->toRenderable();    
    $expected_link['#attributes'] = [
      'target' => '_blank',
    ];
    $expected_link = \Drupal::service('renderer')->renderRoot($expected_link);
    $this->assertContains($expected_link->__toString(), $output);