Add FormBuilderIteratorTest and provide php-ini setting for precision

* Add phpunit to composer.json
* Test-Template-Files from sonata are generated with a php.ini precision setting of 8, so lets run php also with this predefined setting
* FormBuilder of Symfony can be extended (for example: https://github.com/egeloen/ivory-ordered-form/blob/master/src/Builder/OrderedFormBuilder.php) and this will break sonata admin FormBuilderIterator because the private property "children" is not available