Add missing check to unmocked Scheduler warning (#16261)

The unmocked Scheduler warning doesn't actually check if Scheduler
is mocked.

packages/react-dom/src/__tests__/ReactUnmockedSchedulerWarning-test.internal.js

@@ -40,3 +40,17 @@ it('should warn in sync mode', () => {
     ReactDOM.render(<App />, document.createElement('div'));
   }).toWarnDev([]);
 });
+
+it('does not warn if Scheduler is mocked', () => {
+  jest.resetModules();
+  jest.mock('scheduler', () => require('scheduler/unstable_mock'));
+  React = require('react');
+  ReactDOM = require('react-dom');
+  ReactFeatureFlags = require('shared/ReactFeatureFlags');
+  ReactFeatureFlags.warnAboutUnmockedScheduler = true;
+
+  // This should not warn
+  expect(() => {
+    ReactDOM.render(<App />, document.createElement('div'));
+  }).toWarnDev([]);
+});

packages/react-reconciler/src/ReactFiberWorkLoop.js

@@ -2576,11 +2576,14 @@ let didWarnAboutUnmockedScheduler = false;
 
 export function warnIfUnmockedScheduler(fiber: Fiber) {
   if (__DEV__) {
-    if (didWarnAboutUnmockedScheduler === false) {
+    if (
+      didWarnAboutUnmockedScheduler === false &&
+      Scheduler.unstable_flushAllWithoutAsserting === undefined
+    ) {
       if (fiber.mode & BatchedMode || fiber.mode & ConcurrentMode) {
         didWarnAboutUnmockedScheduler = true;
         warningWithoutStack(
-          Scheduler.unstable_flushAllWithoutAsserting !== undefined,
+          false,
           'In Concurrent or Sync modes, the "scheduler" module needs to be mocked ' +
             'to guarantee consistent behaviour across tests and browsers. ' +
             'For example, with jest: \n' +
@@ -2590,7 +2593,7 @@ export function warnIfUnmockedScheduler(fiber: Fiber) {
       } else if (warnAboutUnmockedScheduler === true) {
         didWarnAboutUnmockedScheduler = true;
         warningWithoutStack(
-          null,
+          false,
           'Starting from React v17, the "scheduler" module will need to be mocked ' +
             'to guarantee consistent behaviour across tests and browsers. ' +
             'For example, with jest: \n' +