test: add missing visual tests for board base styles (#9857)

Author: web-padawan

packages/board/test/visual/base/board.test.js

@@ -0,0 +1,36 @@
+import { fixtureSync } from '@vaadin/testing-helpers/dist/fixture.js';
+import { visualDiff } from '@web/test-runner-visual-regression';
+import '../../../src/vaadin-board.js';
+
+describe('board', () => {
+  let element;
+
+  beforeEach(() => {
+    element = fixtureSync(`
+      <vaadin-board>
+        <vaadin-board-row>
+          <div>Cell 1</div>
+          <div>Cell 2</div>
+          <div>Cell 3</div>
+          <div>Cell 4</div>
+        </vaadin-board-row>
+        <vaadin-board-row>
+          <div>Cell 5</div>
+          <div>Cell 6</div>
+          <div>Cell 7</div>
+        </vaadin-board-row>
+        <vaadin-board-row>
+          <div>Cell 8</div>
+          <div>Cell 9</div>
+        </vaadin-board-row>
+        <vaadin-board-row>
+          <div>Cell 10</div>
+        </vaadin-board-row>
+      </vaadin-board>
+    `);
+  });
+
+  it('basic', async () => {
+    await visualDiff(element, 'multiple-rows');
+  });
+});