Method to test if the folder path is valid and points to an existing folder below the Joomla root
Parameters
\SimpleXMLElement $element The SimpleXMLElement object representing the `<field>` tag for the form field object.
mixed$value The form field value to validate.
string$group The field name group control value. This acts as an array container for the field.
For example if the field has name="foo" and the group value is set to "bar" then the
full field name would end up being "bar[foo]".
\Joomla\Registry\Registry $input An optional Registry object with the entire data set to validate against the entire form.
\Joomla\CMS\Form\Form $form The form object for which the field is being tested.
Returns
bool True if the value is valid and points to an existing folder below the Joomla root, false otherwise.