Add private function in feature flags to reset internal JS state for testing (#53056)

Summary:
Pull Request resolved: #53056

Changelog: [internal]

This exposes a utility method in `ReactNativeFeatureFlagsBase` to reset its internal state for testing purposes.

This is intentionally not exposed through `ReactNativeFeatureFlags` to avoid it being used at runtime.

Reviewed By: rshest

Differential Revision: D79639889

fbshipit-source-id: adfb6125d991994c9706d5952d309915fec8f815

Author: rubennorte

packages/react-native/src/private/featureflags/ReactNativeFeatureFlagsBase.js

@@ -18,6 +18,10 @@ import NativeReactNativeFeatureFlags from './specs/NativeReactNativeFeatureFlags
 const accessedFeatureFlags: Set<string> = new Set();
 let overrides: ?ReactNativeFeatureFlagsJsOnlyOverrides;
 
+// This is a list of functions to clear the cached value for each feature flag
+// getter. This is only used in development.
+const clearCachedValuesFns: Array<() => void> = [];
+
 export type Getter<T> = () => T;
 
 // This defines the types for the overrides object, whose methods also receive
@@ -33,6 +37,12 @@ function createGetter<T: boolean | number | string>(
 ): Getter<T> {
   let cachedValue: ?T;
 
+  if (__DEV__) {
+    clearCachedValuesFns.push(() => {
+      cachedValue = undefined;
+    });
+  }
+
   return () => {
     if (cachedValue == null) {
       cachedValue = customValueGetter() ?? defaultValue;
@@ -115,3 +125,12 @@ function maybeLogUnavailableNativeModuleError(configName: string): void {
     );
   }
 }
+
+export function dangerouslyResetForTesting(): void {
+  if (__DEV__) {
+    overrides = null;
+    accessedFeatureFlags.clear();
+    reportedConfigNames.clear();
+    clearCachedValuesFns.forEach(fn => fn());
+  }
+}