test(SideNavDivider): add tests (#13313)

* test(SideNavDivider): add tests

* fix: copyright header year

---------

Co-authored-by: kodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>

packages/react/src/components/UIShell/__tests__/SideNavDivider-test.js

@@ -0,0 +1,20 @@
+/**
+ * Copyright IBM Corp. 2023
+ *
+ * This source code is licensed under the Apache-2.0 license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+import React from 'react';
+import SideNavDivider from '../SideNavDivider';
+import { render } from '@testing-library/react';
+
+describe('SideNavDivider', () => {
+  describe('renders as expected - Component API', () => {
+    it('should support a custom `className` prop on the outermost element', () => {
+      const { container } = render(<SideNavDivider className="custom-class" />);
+
+      expect(container.firstChild).toHaveClass('custom-class');
+    });
+  });
+});