add test for Suspense and Suspense with startTransition (#47714)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/47714

changelog: [internal]

Reviewed By: rshest

Differential Revision: D66093340

fbshipit-source-id: 1db205d42d0570ab9c77ba0363ffac0d2dd81071

jest/integration/runtime/mocks/ReactNativeInternalFeatureFlags.js

@@ -9,4 +9,6 @@
  * @oncall react_native
  */
 
-// Left intentionally empty. All flags are assumed to be false.
+module.exports = {
+  enableFabricCompleteRootInCommitPhase: true,
+};