Update testing based on Failed to parse src "test-file-stub" on `next… (

#29039)

…/image` Error

While learning how to test next.js applications I came across this error when testing components using next/image with an image import

eg: 
```
// /quiz-hero
import Image from 'next/image';
import quizImage from '../../public/undraw-quiz.svg';

// In render 
<Image
              height='91'
              width='198'
              layout='fixed'
              src={quizImage}
              alt='QuizImage'
            />
```

### Error -> 
Failed to parse src "test-file-stub" on `next/image`, if using relative image it must start with a leading slash "/" or be an absolute URL (http:// or https://)

This is fixed by adding a "/" to the beginning of your file-stub module export string. 

This is not an error when you're awaiting an image using async waitFor, But comes up as an error when you're testing a component that holds another component with a "next/image" import.
eg: 
```
// quizHero.test.tsx works like this without change.
const image = await waitFor(() => screen.findByAltText('QuizImage')); 

// but index.test.tsx fails rendering the homepage without the change.
render(<Home />); // Error: Failed to parse src "test-file-stub" on `next/image`...

// with change to __mocks__/fileMock
quizHero.test.tsx //test pass
index.test.tsx //test pass
```
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## Bug

possibly related to -> #26749

## Feature

- [X] Documentation added

## Documentation / Examples

- [X] Make sure the linting passes

docs/testing.md

@@ -319,6 +319,14 @@ These files aren't useful in tests but importing them may cause errors, so we wi
 module.exports = {};
 ```
 
+If you're running into the issue `"Failed to parse src "test-file-stub" on 'next/image'"`, add a '/' to your fileMock.
+
+```json
+// __mocks__/fileMock.js
+
+(module.exports = "/test-file-stub")
+```
+
 For more information on handling static assets, please refer to the [Jest Docs](https://jestjs.io/docs/webpack#handling-static-assets).
 
 **Extend Jest with custom matchers**