There is no guarantee that retrieving form type from ``$formField->vars['block_prefixes'][2]`` is stable.
Better solution is to create form type extension that adds to ``$view->vars`` in ``buildView(FormView $view, FormInterface $form, array $options)`` method information about form type. 
This extension should exist only in tests. 
Example how to get form type name from FormInterface: 

```
echo $form->getConfig()->getType()->getName();
```