[react-native] Add unit test to ReactNativeAttributePayload (#28955)

## Summary

I'm looking at cleaning up some unnecessary manual property flattening
in React Native and wanted to verify this behaviour is working as
expected, where properties from nested objects will always overwrite
properties from the base object.

## How did you test this change?

Unit tests

packages/react-native-renderer/src/__tests__/ReactNativeAttributePayload-test.internal.js

@@ -204,6 +204,19 @@ describe('ReactNativeAttributePayload', () => {
     ).toEqual({foo: null}); // this should ideally be null. heuristic.
   });
 
+  it('handles attributes defined multiple times', () => {
+    const validAttributes = {foo: true, style: {foo: true}};
+    expect(diff({}, {foo: 4, style: {foo: 2}}, validAttributes)).toEqual({
+      foo: 2,
+    });
+    expect(diff({foo: 4}, {style: {foo: 2}}, validAttributes)).toEqual({
+      foo: 2,
+    });
+    expect(diff({style: {foo: 2}}, {foo: 4}, validAttributes)).toEqual({
+      foo: 4,
+    });
+  });
+
   // Function properties are just markers to native that events should be sent.
   it('should convert functions to booleans', () => {
     // Note that if the property changes from one function to another, we don't