test: add missing visual tests for vaadin-overlay base styles (#9831)

Author: web-padawan

packages/overlay/test/visual/base/overlay.test.js

@@ -0,0 +1,28 @@
+import { fixtureSync } from '@vaadin/testing-helpers/dist/fixture.js';
+import { visualDiff } from '@web/test-runner-visual-regression';
+import '../../../src/vaadin-overlay.js';
+
+describe('overlay', () => {
+  let div, element;
+
+  beforeEach(() => {
+    div = document.createElement('div');
+    div.style.height = '100%';
+
+    element = fixtureSync('<vaadin-overlay></vaadin-overlay>', div);
+    element.renderer = (root) => {
+      root.textContent = 'Simple overlay with text';
+    };
+  });
+
+  it('basic', async () => {
+    element.opened = true;
+    await visualDiff(div, 'basic');
+  });
+
+  it('with-backdrop', async () => {
+    element.withBackdrop = true;
+    element.opened = true;
+    await visualDiff(div, 'with-backdrop');
+  });
+});