Added buttonText tests

Testing that the elements with the classes `.fc-button-next`, `.fc-button-nextYear`, `.fc-button-prev` and `.fc-button-prevYear` have empty text when `buttonIcons` is set (by default).

Testing with buttonIcons for `prev`, `next`, `prevYear` and `nextYear` set to `null` that the text of the button coincides exactly with the expected input when language is default, non-default or when the texts have been specified through the `buttonText` property.