chore(cdk-cli-wrapper): fix tests for monorepo (aws#32511)

In a slightly different testing setup, the `jest.spyOn` calls fail with the error:

```
Cannot redefine property: spawnSync
```

This has to do with how the object representing the `fs` module is defined. We can override the module and return a plain object with all the functions, so that the elements can be mocked later on.

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk/cdk-cli-wrapper/test/cdk-wrapper.test.ts

@@ -4,6 +4,9 @@ import { RequireApproval, StackActivityProgress } from '../lib/commands';
 let spawnSyncMock: jest.SpyInstance;
 let spawnMock: jest.SpyInstance;
 
+// Necessary to make the spyOn below work
+jest.mock('child_process', () => ({ __esModule: true, ...jest.requireActual('child_process') }));
+
 beforeEach(() => {
   spawnSyncMock = jest.spyOn(child_process, 'spawnSync').mockReturnValue({
     status: 0,