Skip Performance timeOrigin test on macOS hosts

Summary:
The `timeOrigin` test in `Performance-itest` compares
`performance.now() + performance.timeOrigin` against `Date.now()`. On
macOS this comparison is unreliable because `performance.now()` is
backed by a monotonic clock that does NOT advance while the system is
asleep, so the monotonic time drifts relative to wall time the longer
the machine has been running.

Use the new `Fantom.getRealPlatformOS()` to skip just this test on macOS
hosts while keeping it active on other platforms. A comment explains
why the test cannot be run there.

Changelog: [Internal]

Differential Revision: D106669674

Author: rubennorte

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

@@ -10,6 +10,8 @@
 
 import '@react-native/fantom/src/setUpDefaultReactNativeEnvironment';
 
+import * as Fantom from '@react-native/fantom';
+
 describe('Performance', () => {
   it('does NOT allow creating instances of Performance directly', () => {
     expect(() => {
@@ -40,7 +42,24 @@ describe('Performance', () => {
   });
 
   describe('timeOrigin', () => {
-    it('allows moving timestamps to Unix epoch', () => {
+    it('is a positive number that does not change between calls', () => {
+      const first = performance.timeOrigin;
+      const second = performance.timeOrigin;
+
+      expect(typeof first).toBe('number');
+      expect(first).toBeGreaterThan(0);
+      expect(second).toBe(first);
+    });
+
+    // On macOS, `performance.now()` is backed by a monotonic clock that does
+    // NOT include time spent while the system is asleep. This causes the
+    // monotonic time to drift relative to wall time (`Date.now()`) the longer
+    // the machine has been running, so we can't reliably compare
+    // `performance.now() + performance.timeOrigin` against `Date.now()` like
+    // we can on other platforms.
+    const itIfNotMacOS = Fantom.getRealPlatformOS() === 'macos' ? it.skip : it;
+
+    itIfNotMacOS('allows moving timestamps to Unix epoch', () => {
       // We need to truncate timestamps because `Date.now()` only provides
       // integer millisecond precision.
       const adjustedMonotonicTime = Math.trunc(