Fix for issue #680: Add testing for the bug

Refactor \FeatureContext::assertBackendLoggedOut to also ensure the
login page displays the login form.

Add a new testing step definition that logs out via the logout url
rather than fast logout.

Add a new test to backend_login.feature to ensure logout via the url is
working correctly.

features/backend_login.feature

@@ -8,8 +8,14 @@ Feature: Backend login/logout
     Given I am logged in as a user with the "authenticated user" role
     Then I should be logged in on the backend
 
-  Scenario: Logout on the backend
+  Scenario: Logout on the backend via fast logout
     Given I am logged in as a user with the "authenticated user" role
     And I am logged in on the backend
     When I log out
     Then I should be logged out on the backend
+
+  Scenario: Logout on the backend via url
+    Given I am logged in as a user with the "authenticated user" role
+    And I am logged in on the backend
+    When I log out via the logout url
+    Then I should be logged out on the backend

features/bootstrap/FeatureContext.php

@@ -229,6 +229,20 @@ public function assertBackendLoggedOut()
         if (!\Drupal::currentUser()->isAnonymous()) {
             throw new \LogicException('User is still logged in on the backend.');
         }
+        // Visit login page and ensure login form is present.
+        $this->getSession()->visit($this->locatePath($this->getDrupalText('login_url')));
+        $element = $this->getSession()->getPage();
+        $element->fillField($this->getDrupalText('username_field'), 'foo');
+    }
+
+    /**
+     * Logs out via the logout url rather than fast logout.
+     *
+     * @Then I log out via the logout url
+     */
+    public function logoutViaUrl()
+    {
+      $this->logout(false);
     }
 
   /**