Use act in fuzz tester to flush expired work (facebook#21108)

* Add failing hard-coded fuzz test

Caught in CI by the fuzz tester.

Related to expired updates.

* Use `act` in fuzz tester to flush expired work

Expired work gets scheduled in a microtask now, so we need to use `act`
to flush it.

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

@@ -432,7 +432,9 @@ describe('ReactIncrementalTriangle', () => {
         assertConsistentTree(activeLeafIndices);
       }
       // Flush remaining work
-      Scheduler.unstable_flushAllWithoutAsserting();
+      ReactNoop.act(() => {
+        Scheduler.unstable_flushAllWithoutAsserting();
+      });
       assertConsistentTree(activeLeafIndices, expectedCounterAtEnd);
     }
 
@@ -562,6 +564,12 @@ ${formatActions(actions)}
         ['b', flush(7)],
         ['c', toggle(0)],
       );
+
+      simulateMultipleRoots(
+        ['c', step(1)],
+        ['c', expire(5000)],
+        ['b', toggle(1)],
+      );
     });
 
     it('generative tests', () => {