introduce RCTTestUtils (#37810)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/37810

Changelog: [Internal]

in this change, i introduce a helper file for swizzling selectors that can be used for building custom shims and fakes. i tried putting this in the cxx target, but i struggled a lot with managing the dependencies and understanding the extra behavior that was happening from being in a `rn_xplat_cxx_library`. this file is also not obj-c++, so i feel like we should decouple if possible.

it does seem like this is a new practice i'm introducing, so let me know if there's something that i'm missing.

Reviewed By: cipolleschi

Differential Revision: D46507803

fbshipit-source-id: 9be63d992b9838e9a98ee0c1c6d684fc59c3fc80

packages/react-native/ReactCommon/react/test_utils/ios/RCTSwizzleHelpers.h

@@ -0,0 +1,19 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#import <Foundation/Foundation.h>
+
+#import <React/RCTDefines.h>
+
+RCT_EXTERN_C_BEGIN
+
+void RCTSwizzleInstanceSelector(
+    Class targetClass,
+    Class swizzleClass,
+    SEL selector);
+
+RCT_EXTERN_C_END

packages/react-native/ReactCommon/react/test_utils/ios/RCTSwizzleHelpers.m

@@ -0,0 +1,17 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#import "RCTSwizzleHelpers.h"
+
+#import <objc/runtime.h>
+
+void RCTSwizzleInstanceSelector(Class targetClass, Class swizzleClass, SEL selector)
+{
+  Method originalMethod = class_getInstanceMethod(targetClass, selector);
+  Method swizzleMethod = class_getInstanceMethod(swizzleClass, selector);
+  method_exchangeImplementations(originalMethod, swizzleMethod);
+}