code-to-explain-code test in MultiCheckboxTest to check multi selected 

I think it may be a bit confusing on how an `$optionValue` and `$selectedOptions` work together with `Zend\Form\Element\MultiCheckbox` for those who are not using `Zend\Form\View\Helper\FormMultiCheckbox`  I hope this test will explain this issue in code and that people will feel more comfortable creating their own view helpers in the future.

Thank you for you consideration!