Fix broken ReactNativeFeatureFlagsTest test on iOS (#42699)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/42699

Changelog: [internal]

The test was fine on C++. The reason is probably that C++ destroys the process between tests (effectively resetting singletons) while iOS doesn't.

This fixes the test by implementing a correct `TearDown` method to reset the flags.

Reviewed By: rshest

Differential Revision: D53178474

fbshipit-source-id: 6a0f67f1a59fe47f73a495344d4c0daa8eafa3c4

packages/react-native/ReactCommon/react/featureflags/tests/ReactNativeFeatureFlagsTest.cpp

@@ -28,6 +28,10 @@ class ReactNativeFeatureFlagsTest : public testing::Test {
   void SetUp() override {
     overrideAccessCount = 0;
   }
+
+  void TearDown() override {
+    ReactNativeFeatureFlags::dangerouslyReset();
+  }
 };
 
 TEST_F(ReactNativeFeatureFlagsTest, providesDefaults) {