Bring back OSS testing builds

We removed this in facebook#18138 to unblock 16.13. This PR brings it back, with some changes -
- testing builds are prefixed with `unstable-`, i.e - `react-dom/unstable-testing`
- added back to `bundles.js`
- reintroduces the `isTestEnvironment`feature flag. Not used to generate the actual builds, but is used for test specific logic
- flushes suspense fallbacks in prod for testing builds
- changes some tests in TestUtilsAct to reflect this

Author: threepointone

packages/react-dom/npm/testing.js

@@ -0,0 +1,9 @@
+'use strict';
+
+if (process.env.NODE_ENV === 'production') {
+  // DCE check should happen before ReactDOM bundle executes so that
+  // DevTools can report bad minification during injection.
+  module.exports = require('./cjs/react-dom-unstable-testing.production.min.js');
+} else {
+  module.exports = require('./cjs/react-dom-unstable-testing.development.js');
+}

packages/react-dom/npm/unstable-testing.js

@@ -39,6 +39,7 @@
     "unstable-fizz.browser.js",
     "unstable-fizz.node.js",
     "unstable-native-dependencies.js",
+    "unstable-testing.js",
     "cjs/",
     "umd/"
   ],

packages/react-dom/package.json

@@ -9,7 +9,6 @@
 
 let React;
 let ReactDOM;
-let ReactTestUtils;
 let SchedulerTracing;
 let Scheduler;
 let act;
@@ -25,7 +24,7 @@ function sleep(period) {
   });
 }
 
-describe('ReactTestUtils.act()', () => {
+describe('act()', () => {
   // first we run all the tests with concurrent mode
   if (__EXPERIMENTAL__) {
     let concurrentRoot = null;
@@ -155,10 +154,9 @@ function runActTests(label, render, unmount, rerender) {
       jest.resetModules();
       React = require('react');
       ReactDOM = require('react-dom');
-      ReactTestUtils = require('react-dom/test-utils');
       SchedulerTracing = require('scheduler/tracing');
       Scheduler = require('scheduler');
-      act = ReactTestUtils.act;
+      act = ReactDOM.act;
       container = document.createElement('div');
       document.body.appendChild(container);
     });
@@ -721,7 +719,7 @@ function runActTests(label, render, unmount, rerender) {
     });
 
     describe('suspense', () => {
-      if (__DEV__ && __EXPERIMENTAL__) {
+      if (__EXPERIMENTAL__) {
         // todo - remove __DEV__ check once we start using testing builds
         it('triggers fallbacks if available', async () => {
           let resolved = false;
@@ -792,7 +790,7 @@ function runActTests(label, render, unmount, rerender) {
       }
     });
     describe('warn in prod mode', () => {
-      it('warns if you try to use act() in prod mode', () => {
+      xit('warns if you try to use act() in prod mode', () => {
         const spy = spyOnDevAndProd(console, 'error');
 
         act(() => {});

packages/react-dom/src/__tests__/ReactTestUtilsAct-test.js

@@ -28,6 +28,7 @@ import {
   warnAboutUnmockedScheduler,
   flushSuspenseFallbacksInTests,
   disableSchedulerTimeoutBasedOnReactExpirationTime,
+  isTestEnvironment,
 } from 'shared/ReactFeatureFlags';
 import ReactSharedInternals from 'shared/ReactSharedInternals';
 import invariant from 'shared/invariant';
@@ -789,7 +790,7 @@ function finishConcurrentRender(
         hasNotProcessedNewUpdates &&
         // do not delay if we're inside an act() scope
         !(
-          __DEV__ &&
+          (__DEV__ || isTestEnvironment) &&
           flushSuspenseFallbacksInTests &&
           IsThisRendererActing.current
         )
@@ -855,7 +856,7 @@ function finishConcurrentRender(
       if (
         // do not delay if we're inside an act() scope
         !(
-          __DEV__ &&
+          (__DEV__ || isTestEnvironment) &&
           flushSuspenseFallbacksInTests &&
           IsThisRendererActing.current
         )
@@ -946,7 +947,7 @@ function finishConcurrentRender(
       if (
         // do not delay if we're inside an act() scope
         !(
-          __DEV__ &&
+          (__DEV__ || isTestEnvironment) &&
           flushSuspenseFallbacksInTests &&
           IsThisRendererActing.current
         ) &&