MDL-66979 mod_data: Improve behat chart data step

Charts data visibility is more reliable when checked via the
aria-expanded attribute.

The isVisible attribute can cause some failures under Firefox.

mod/feedback/tests/behat/behat_mod_feedback.php

@@ -165,9 +165,7 @@ class behat_mod_feedback extends behat_base {
         // If chart data is not visible then expand.
         $node = $this->get_selected_node("xpath_element", $charttabledataxpath);
         if ($node) {
-            if (!$node->isVisible()) {
-                // Focus on node, before checking if it's visible.
-                $node->focus();
+            if ($node->getAttribute('aria-expanded') === 'false') {
                 $this->execute('behat_general::i_click_on_in_the', array(
                     get_string('showchartdata'),
                     'link',