[tests] disableLegacyMode in test-renderer (facebook#35002)

500 tests failed from not using async act. Will fix the tests and then
re-land this.

Author: rickhanlonii

packages/shared/forks/ReactFeatureFlags.test-renderer.native-fb.js

@@ -16,7 +16,7 @@ export const disableCommentsAsDOMContainers = true;
 export const disableInputAttributeSyncing = false;
 export const disableLegacyContext = false;
 export const disableLegacyContextForFunctionComponents = false;
-export const disableLegacyMode = true;
+export const disableLegacyMode = false;
 export const disableSchedulerTimeoutInWorkLoop = false;
 export const disableTextareaChildren = false;
 export const enableAsyncDebugInfo = false;