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.

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 (