Can't mock nested field resolvers

### Issue workflow progress

_Progress of the issue based on the [Contributor Workflow](https://github.com/the-guild-org/Stack/blob/master/CONTRIBUTING.md#a-typical-contributor-workflow)_

- [x] 1. The issue provides a reproduction available on Github, Stackblitz or CodeSandbox
> Make sure to fork this template and run `yarn generate` in the terminal.
>
> Please make sure the GraphQL Tools package versions under `package.json` matches yours.
- [x] 2. A failing test has been provided
- [ ] 3. A local solution has been provided
- [ ] 4. A pull request is pending review

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**Describe the bug**

When mocking a nested root query, I can override any simple field, but overriding resolver fields doesn't work.

**To Reproduce**

[Here is a test showing the problem](https://github.com/sunesimonsen/graphql-tools/blob/ssimonsen/mock-nested-root-query/packages/mock/tests/mocking-compatibility.spec.ts#L580-L611)

You can run the test in my fork, or just copy over the test.

**Expected behavior**

I expect `returnStringArgument` to have the value `Overridden`, but I get the default string value `Hello World`.

**Environment:**

- OS: OSX
- `@graphql-tools/mock`:
- NodeJS: v16.18.0

**Additional context**

I'm trying to fix a problem we are seeing in another tool [graphql-fakester](https://github.com/sunesimonsen/graphql-fakester/issues/103)

Start with the reproduction in packages/mock/tests/mocking-compatibility.spec.ts, specifically the test linked at lines 580-611, and run it after copying it from the referenced fork. Done means the nested resolver override makes returnStringArgument equal "Overridden" instead of the default "Hello World".

Written by the indexing model from the issue text.