Add a simple waitForApplication for servlet integration tests

This waitForApplication checks if an UI element exists on the page. If
not, then it will wait 10 seconds in order for it to appear. This should
reduce the amount of false negatives in server tests.

Change-Id: I37a398e60c247920fd56fc05b747e0f93f3f2dd2

uitest/src/com/vaadin/tests/integration/AbstractIntegrationTest.java

@@ -15,6 +15,8 @@
  */
 package com.vaadin.tests.integration;
 
+import com.vaadin.testbench.By;
+import com.vaadin.testbench.elements.UIElement;
 import com.vaadin.testbench.parallel.TestNameSuffix;
 import com.vaadin.tests.tb3.PrivateTB3Configuration;
 
@@ -37,4 +39,17 @@ public abstract class AbstractIntegrationTest extends PrivateTB3Configuration {
         return deploymentUrl;
     }
 
+    @Override
+    protected void openTestURL() {
+        super.openTestURL();
+
+        waitForApplication();
+    }
+
+    protected void waitForApplication() {
+        if (!isElementPresent(UIElement.class)) {
+            // Wait for UI element.
+            waitForElementPresent(By.vaadin("//com.vaadin.ui.UI"));
+        }
+    }
 }