Add Invalid Username/Password to users sample

gh-49

samples/users/src/integration-test/groovy/sample/UserTests.groovy

@@ -21,6 +21,26 @@ class UserTests extends GebReportingSpec {
         !username
     }
 
+    def 'invalid login'() {
+        setup:
+        def user = 'rob'
+        when:
+        login(user, user+'invalid')
+        then:
+        !username
+        error == 'Invalid username / password. Please ensure the username is the same as the password.'
+    }
+
+    def 'empty username'() {
+        setup:
+        def user = ''
+        when:
+        login(user, user)
+        then:
+        !username
+        error == 'Invalid username / password. Please ensure the username is the same as the password.'
+    }
+
     def 'login single user'() {
         setup:
         def user = 'rob'

samples/users/src/integration-test/groovy/sample/pages/HomePage.groovy

@@ -12,6 +12,7 @@ class HomePage extends Page {
     static at = { assert driver.title == 'Demonstrates Multi User Log In'; true}
     static content = {
         navLink { $('#navLink') }
+        error { $('#error').text() }
         form { $('form') }
         username(required:false) { $('#un').text() }
         logout(required:false) { $('#logout') }