Stabilize FormValidationTest

Current API is flawed for asserting form validation.
Matchers should be able to retrieve state from selenium, without knowing
what you are looking for you don't know what should be on the page.

Added a hacky sleep to stabilize test suite rather than adjusting the
API

src/test/java/core/FormValidationTest.java

@@ -23,13 +23,12 @@
  */
 package core;
 
-import hudson.util.VersionNumber;
 import org.jenkinsci.test.acceptance.junit.AbstractJUnitTest;
 import org.jenkinsci.test.acceptance.po.FormValidation;
 import org.jenkinsci.test.acceptance.po.JenkinsConfig;
 import org.jenkinsci.test.acceptance.po.ListView;
 import org.junit.Test;
-import org.openqa.selenium.Alert;
+import org.openqa.selenium.Keys;
 
 import static org.hamcrest.MatcherAssert.assertThat;
 import static org.hamcrest.Matchers.equalTo;
@@ -64,6 +63,8 @@ private void jsValidation() {
         assertThat(formValidation, silent());
 
         c.numExecutors.set(-16);
+        c.numExecutors.resolve().sendKeys(Keys.TAB);
+        c.numExecutors.sleep(100L);
         formValidation = c.numExecutors.getFormValidation();
 
         String errorMessage = "Not a non-negative integer";