allow nested act()s from different renderers (#16039)

* allow nested `act()`s from different renderers

There are usecases where multiple renderers need to oprate inside an act() scope
- ReactDOM.render being used inside another component tree. The parent component will be rendered using ReactTestRenderer.create for a snapshot test or something.
- a ReactDOM instance interacting with a ReactTestRenderer instance (like for the new devtools)

This PR changes the way the acting sigils operate to allow for this. It keeps 2 booleans, one attached to React, one attached to the renderer. act() changes these values, and the workloop reads them to decide what warning to trigger.

I also renamed shouldWarnUnactedUpdates to warnsIfNotActing

* s/ReactIsActing/IsSomeRendererActing and s/ReactRendererIsActing/IsThisRendererActing

src/ReactTestHostConfig.js

@@ -217,7 +217,7 @@ export function createTextInstance(
 }
 
 export const isPrimaryRenderer = false;
-export const shouldWarnUnactedUpdates = true;
+export const warnsIfNotActing = true;
 
 export const scheduleTimeout = setTimeout;
 export const cancelTimeout = clearTimeout;

src/ReactTestRendererAct.js

@@ -11,15 +11,15 @@ import type {Thenable} from 'react-reconciler/src/ReactFiberWorkLoop';
 import {
   batchedUpdates,
   flushPassiveEffects,
-  ReactActingRendererSigil,
+  IsThisRendererActing,
 } from 'react-reconciler/inline.test';
 import ReactSharedInternals from 'shared/ReactSharedInternals';
 import warningWithoutStack from 'shared/warningWithoutStack';
 import {warnAboutMissingMockScheduler} from 'shared/ReactFeatureFlags';
 import enqueueTask from 'shared/enqueueTask';
 import * as Scheduler from 'scheduler';
 
-const {ReactCurrentActingRendererSigil} = ReactSharedInternals;
+const {IsSomeRendererActing} = ReactSharedInternals;
 
 // this implementation should be exactly the same in
 // ReactTestUtilsAct.js, ReactTestRendererAct.js, createReactNoop.js
@@ -67,17 +67,21 @@ let actingUpdatesScopeDepth = 0;
 
 function act(callback: () => Thenable) {
   let previousActingUpdatesScopeDepth = actingUpdatesScopeDepth;
-  let previousActingUpdatesSigil;
+  let previousIsSomeRendererActing;
+  let previousIsThisRendererActing;
   actingUpdatesScopeDepth++;
   if (__DEV__) {
-    previousActingUpdatesSigil = ReactCurrentActingRendererSigil.current;
-    ReactCurrentActingRendererSigil.current = ReactActingRendererSigil;
+    previousIsSomeRendererActing = IsSomeRendererActing.current;
+    previousIsThisRendererActing = IsSomeRendererActing.current;
+    IsSomeRendererActing.current = true;
+    IsThisRendererActing.current = true;
   }
 
   function onDone() {
     actingUpdatesScopeDepth--;
     if (__DEV__) {
-      ReactCurrentActingRendererSigil.current = previousActingUpdatesSigil;
+      IsSomeRendererActing.current = previousIsSomeRendererActing;
+      IsThisRendererActing.current = previousIsThisRendererActing;
       if (actingUpdatesScopeDepth > previousActingUpdatesScopeDepth) {
         // if it's _less than_ previousActingUpdatesScopeDepth, then we can assume the 'other' one has warned
         warningWithoutStack(