Add test for alert dismissal

This also fixes an edge case in the `useLocalStorageSpy` helper, which
didn't initialise the localStorage getter properly in the case that
earlier, unrelated `describe` blocks in the same suite rely on real
`localStorage` behaviour, or just don't need it to be mocked.

ee/spec/frontend/security_configuration/components/app_spec.js

@@ -4,6 +4,7 @@ import { GlAlert, GlLink } from '@gitlab/ui';
 import SecurityConfigurationApp from 'ee/security_configuration/components/app.vue';
 import ManageFeature from 'ee/security_configuration/components/manage_feature.vue';
 import stubChildren from 'helpers/stub_children';
+import { useLocalStorageSpy } from 'helpers/local_storage_helper';
 import LocalStorageSync from '~/vue_shared/components/local_storage_sync.vue';
 import { generateFeatures } from './helpers';
 
@@ -127,6 +128,35 @@ describe('Security Configuration App', () => {
         }
       },
     );
+
+    describe('dismissing the alert', () => {
+      useLocalStorageSpy();
+
+      beforeEach(() => {
+        createComponent({
+          propsData: {
+            gitlabCiPresent: false,
+            autoDevopsEnabled: false,
+            canEnableAutoDevops: true,
+          },
+          stubs: {
+            LocalStorageSync,
+          },
+        });
+
+        getAlert().vm.$emit('dismiss');
+      });
+
+      it('hides the alert', () => {
+        expect(getAlert().exists()).toBe(false);
+      });
+
+      it('saves dismissal in localStorage', () => {
+        expect(localStorage.setItem.mock.calls).toEqual([
+          [SecurityConfigurationApp.autoDevopsAlertStorageKey, 'true'],
+        ]);
+      });
+    });
   });
 
   describe('features table', () => {

spec/frontend/helpers/local_storage_helper.js

@@ -10,7 +10,7 @@
  */
 const useLocalStorage = fn => {
   const origLocalStorage = window.localStorage;
-  let currentLocalStorage;
+  let currentLocalStorage = origLocalStorage;
 
   Object.defineProperty(window, 'localStorage', {
     get: () => currentLocalStorage,

spec/frontend/helpers/local_storage_helper_spec.js

 import { useLocalStorageSpy } from './local_storage_helper';
 
-useLocalStorageSpy();
+describe('block before helper is installed', () => {
+  it('should leave original localStorage intact', () => {
+    expect(localStorage.getItem).toEqual(expect.any(Function));
+    expect(jest.isMockFunction(localStorage.getItem)).toBe(false);
+  });
+});
 
 describe('localStorage helper', () => {
+  useLocalStorageSpy();
+
   it('mocks localStorage but works exactly like original localStorage', () => {
     localStorage.setItem('test', 'testing');
     localStorage.setItem('test2', 'testing');