Migrate all feature flags to pragmas (#48098)

Summary:
Pull Request resolved: #48098

Changelog: [internal]

TSIA. No more manual feature flag overrides in Fantom tests :D

Reviewed By: sammy-SC

Differential Revision: D66760120

fbshipit-source-id: a0493d6ca57f4fdad33a0667e3af99ed0f0b66ca

Author: rubennorte

packages/react-native/Libraries/ReactNative/ReactFabricPublicInstance/__tests__/ReactFabricPublicInstance-Legacy-itest.js

@@ -7,6 +7,7 @@
  * @flow strict-local
  * @format
  * @oncall react_native
+ * @fantom_flags enableAccessToHostTreeInFabric:false
  */
 
 import setUpReactFabricPublicInstanceFantomTests from './setUpReactFabricPublicInstanceFantomTests';

packages/react-native/Libraries/ReactNative/ReactFabricPublicInstance/__tests__/ReactFabricPublicInstance-Modern-itest.js

@@ -7,9 +7,9 @@
  * @flow strict-local
  * @format
  * @oncall react_native
+ * @fantom_flags enableAccessToHostTreeInFabric:true
  */
 
-import './setUpFeatureFlags';
 import setUpReactFabricPublicInstanceFantomTests from './setUpReactFabricPublicInstanceFantomTests';
 
 setUpReactFabricPublicInstanceFantomTests({isModern: true});

packages/react-native/Libraries/ReactNative/ReactFabricPublicInstance/__tests__/setUpFeatureFlags.js

@@ -7,6 +7,7 @@
  * @flow strict-local
  * @format
  * @oncall react_native
+ * @fantom_flags enableAccessToHostTreeInFabric:true
  */
 
 /* eslint-disable lint/sort-imports */
@@ -19,7 +20,6 @@ import ReactNativeElement from '../../dom/nodes/ReactNativeElement';
 import IntersectionObserverEntry from '../IntersectionObserverEntry';
 import * as React from 'react';
 
-import './setUpFeatureFlags';
 import '../../../../../Libraries/Core/InitializeCore.js';
 
 import ScrollView from '../../../../../Libraries/Components/ScrollView/ScrollView';

packages/react-native/src/private/webapis/intersectionobserver/__tests__/IntersectionObserver-itest.js

@@ -7,6 +7,7 @@
  * @flow strict-local
  * @format
  * @oncall react_native
+ * @fantom_flags enableAccessToHostTreeInFabric:true
  */
 
 import type MutationObserverType from '../MutationObserver';
@@ -18,7 +19,6 @@ import ReactNativeElement from '../../dom/nodes/ReactNativeElement';
 import nullthrows from 'nullthrows';
 import * as React from 'react';
 
-import './setUpFeatureFlags';
 import '../../../../../Libraries/Core/InitializeCore.js';
 
 declare const MutationObserver: Class<MutationObserverType>;