test: add Lumo visual tests for standalone login-form (#9688)

Author: web-padawan

packages/login/test/visual/lumo/login-form.test.js

@@ -0,0 +1,28 @@
+import { fixtureSync } from '@vaadin/testing-helpers';
+import { visualDiff } from '@web/test-runner-visual-regression';
+import '@vaadin/vaadin-lumo-styles/props.css';
+import '@vaadin/vaadin-lumo-styles/global.css';
+import '@vaadin/vaadin-lumo-styles/components/login.css';
+import '../common.js';
+import '../../../vaadin-login-form.js';
+
+describe('login-form', () => {
+  let div, element;
+
+  beforeEach(() => {
+    div = document.createElement('div');
+    div.style.display = 'inline-block';
+    div.style.padding = '10px';
+    element = fixtureSync('<vaadin-login-form></vaadin-login-form>', div);
+  });
+
+  it('basic', async () => {
+    element.opened = true;
+    await visualDiff(div, 'basic');
+  });
+
+  it('error', async () => {
+    element.error = true;
+    await visualDiff(div, 'error');
+  });
+});