Add DevSettings to jest preset (#29223)

Summary:
Provide a mocked `DevSettings` implementation for jest.

## Changelog

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[General] [Added] - Add mock for `DevSettings` to jest preset
Pull Request resolved: https://github.com/facebook/react-native/pull/29223

Test Plan: None

Reviewed By: cpojer

Differential Revision: D22279962

Pulled By: TheSavior

fbshipit-source-id: 667cecb0a558a4267564702ee9d30380756bdd92

jest/setup.js

@@ -211,6 +211,10 @@ jest
         };
       },
     },
+    DevSettings: {
+      addMenuItem: jest.fn(),
+      reload: jest.fn(),
+    },
     ImageLoader: {
       getSize: jest.fn(url => Promise.resolve({width: 320, height: 240})),
       prefetchImage: jest.fn(),