Create placeholder for User Timing API tests

Summary:
Changelog: [internal]

Just a placeholder to add new tests in the future.

Differential Revision: D68093342

packages/react-native/src/private/webapis/performance/__tests__/UserTimingAPI-itest.js

@@ -0,0 +1,71 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow strict-local
+ * @format
+ * @oncall react_native
+ * @fantom_flags enableLongTaskAPI:true
+ */
+
+import setUpPerformanceObserver from '../../../setup/setUpPerformanceObserver';
+import ensureInstance from '../../../utilities/ensureInstance';
+import {PerformanceMark} from '../UserTiming';
+import {runWorkLoop} from '@react-native/fantom';
+
+import '../../../../../Libraries/Core/InitializeCore.js';
+
+setUpPerformanceObserver();
+
+describe('User Timing API', () => {
+  describe('performance.mark()', () => {
+    it('reports marks to observers', () => {
+      const callback = jest.fn();
+
+      const observer = new PerformanceObserver(callback);
+      observer.observe({type: 'mark'});
+
+      expect(callback).not.toHaveBeenCalled();
+
+      const mark1Detail = Symbol('mark1Detail');
+      performance.mark('mark1', {
+        startTime: 100,
+        detail: mark1Detail,
+      });
+
+      const mark2Detail = Symbol('mark2Detail');
+      performance.mark('mark2', {
+        startTime: 200,
+        detail: mark2Detail,
+      });
+
+      expect(callback).not.toHaveBeenCalled();
+
+      runWorkLoop();
+
+      expect(callback).toHaveBeenCalledTimes(1);
+
+      const entries = callback.mock.lastCall[0].getEntries();
+      expect(entries.length).toBe(2);
+
+      const mark1 = ensureInstance(entries[0], PerformanceMark);
+      const mark2 = ensureInstance(entries[1], PerformanceMark);
+
+      expect(mark1.entryType).toBe('mark');
+      expect(mark1.name).toBe('mark1');
+      expect(mark1.startTime).toBe(100);
+      expect(mark1.duration).toBe(0);
+      // This doesn't work through PerformanceObserver yet
+      // expect(mark1.detail).toBe(mark1Detail);
+
+      expect(mark2.entryType).toBe('mark');
+      expect(mark2.name).toBe('mark2');
+      expect(mark2.startTime).toBe(200);
+      expect(mark2.duration).toBe(0);
+      // This doesn't work through PerformanceObserver yet
+      // expect(mark2.detail).toBe(mark2Detail);
+    });
+  });
+});