Check correct commit phase props in fuzz tester (facebook#14129)

Adds a check to the existing fuzz tester to confirm that the props are
set to the latest values in the commit phase. Only checks
componentDidUpdate; we already have unit tests for the other lifecycles,
so I think this is good enough. This is only a redundancy.

Author: acdlite

packages/react-reconciler/src/__tests__/ReactIncrementalTriangle-test.internal.js

@@ -189,6 +189,7 @@ describe('ReactIncrementalTriangle', () => {
           leafTriangles.push(this);
         }
         this.state = {isActive: false};
+        this.child = React.createRef(null);
       }
       activate() {
         this.setState({isActive: true});
@@ -203,6 +204,14 @@ describe('ReactIncrementalTriangle', () => {
           this.state.isActive !== nextState.isActive
         );
       }
+      componentDidUpdate() {
+        if (this.child.current !== null) {
+          const {prop: currentCounter} = JSON.parse(this.child.current.prop);
+          if (this.props.counter !== currentCounter) {
+            throw new Error('Incorrect props in lifecycle');
+          }
+        }
+      }
       render() {
         if (yieldAfterEachRender) {
           ReactNoop.yield(this);
@@ -228,7 +237,7 @@ describe('ReactIncrementalTriangle', () => {
                       activeDepthProp,
                       activeDepthContext,
                     });
-                    return <span prop={output} />;
+                    return <span ref={this.child} prop={output} />;
                   }
 
                   return (