Update testing.md (#29954)

I've improved jest configuration that :
1. Starts collecting test coverage
1. Does not collect coverage for derived files or coverage info itself
1. Uses coverage provider that is based on node itself.



## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`

## Documentation / Examples

- [ ] Make sure the linting passes


Co-authored-by: JJ Kasper <22380829+ijjk@users.noreply.github.com>

docs/testing.md

@@ -311,10 +311,17 @@ Here are the recommended options to configure Jest for Next.js:
 ```jsx
 // jest.config.js
 module.exports = {
+  collectCoverage: true,
+  // on node 14.x coverage provider v8 offers good speed and more or less good report
+  coverageProvider: 'v8',
   collectCoverageFrom: [
     '**/*.{js,jsx,ts,tsx}',
     '!**/*.d.ts',
     '!**/node_modules/**',
+    '!<rootDir>/out/**',
+    '!<rootDir>/.next/**',
+    '!<rootDir>/*.config.js',
+    '!<rootDir>/coverage/**',
   ],
   moduleNameMapper: {
     // Handle CSS imports (with CSS modules)