[Misc] Wait more thoroughly for the tree to be loaded

This is an attempt to reduce flickering in tests that might be caused by
proceeding too quickly with loading other tree nodes while the initial
tree nodes are still loading. Then possibly the selection of the root
node might not be cleared correctly before selecting the desired node as
the root node is automatically selected during the initial load.

(cherry picked from commit 8e0c1cd9)

xwiki-platform-core/xwiki-platform-tree/xwiki-platform-tree-test-pageobjects/src/main/java/org/xwiki/tree/test/po/TreeElement.java

@@ -26,10 +26,8 @@
 
 import org.openqa.selenium.By;
 import org.openqa.selenium.Keys;
-import org.openqa.selenium.WebDriver;
 import org.openqa.selenium.WebElement;
 import org.openqa.selenium.interactions.Actions;
-import org.openqa.selenium.support.ui.ExpectedCondition;
 import org.xwiki.test.ui.po.BaseElement;
 
 /**
@@ -117,15 +115,12 @@ public TreeElement waitForIt()
         // being initialized, so its presence guarantees that the tree initialization has started.
         getDriver().waitUntilElementIsVisible(this.element, By.cssSelector(".jstree-container-ul"));
         // Wait for the root node to be loaded.
-        getDriver().waitUntilCondition(new ExpectedCondition<Boolean>()
-        {
-            @Override
-            public Boolean apply(WebDriver driver)
-            {
-                // The tree element is marked as busy while the tree nodes are being loaded.
-                return !Boolean.valueOf(element.getAttribute("aria-busy"));
-            }
-        });
+        getDriver().waitUntilCondition(driver ->
+            // The tree element is marked as busy while the tree nodes are being loaded.
+            !Boolean.parseBoolean(this.element.getAttribute("aria-busy"))
+                // Check if there is any descendant of the element that is marked as busy.
+                && this.element.findElements(By.cssSelector("[aria-busy = 'true']")).isEmpty()
+        );
         return this;
     }