Update js file to add testability to the app

ee/app/assets/javascripts/boards/toggle_focus.js

@@ -31,6 +31,7 @@ export default (ModalStore, boardsStore, $boardApp) => {
         <a
           href="#"
           class="btn btn-default has-tooltip prepend-left-10 js-focus-mode-btn"
+          data-qa-selector="focus_mode_button"
           role="button"
           aria-label="Toggle focus mode"
           title="Toggle focus mode"