[DomCrawler] check for the correct field type

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11689
| License       | MIT
| Doc PR        | 

HTML allow to define different form fields with the same name. Imagine the following form:

```html
    <html>
        <body>
            <form action="/">
                <input type="hidden" name="option" value="default">
                <input type="radio" name="option" value="A">
                <input type="radio" name="option" value="B">
                <input type="hidden" name="settings[1]" value="0">
                <input type="checkbox" name="settings[1]" value="1" id="setting-1">
                <button>klickme</button>
            </form>
        </body>
    </html>
```

Since the `FormFieldRegistry` can only handle one field per name, the hidden field option is registered first before the radio field with the same name is evaluated. Thus, the `FormFieldRegistry` returns an `InputFormField` instance on which the `addChoices()` method can not be called.
you should add an assertion in this test. I suggest

```php
$this->assertInstanceOf('...\ChoiceFormField', $form->get('option'));
```