[Documentation] Update CONTRIBUTING.md with current test execution instructions

Summary

The CONTRIBUTING.md file contains outdated test execution instructions that don't fully match current project setup.

Current Issues

1. Test command inconsistency:

   • CONTRIBUTING.md mentions npm test and npm run test-localization
   • package.json shows these scripts exist but additional test commands are available
   • No mention of test:coverage command that exists in gulpfile.js

2. Smoke test documentation:

   • Says to run yarn smoke-tests but package.json uses npm, not yarn
   • References Launch All Smoke Tests command but no details about setup requirements
   • Links to test/smoke/docs/run-locally.md for setup but doesn't explain prerequisites

3. Missing test information:

   • No mention of test-no-build option
   • No explanation of watch mode (watch-build-test)
   • No coverage of gulp test vs npm test differences

Current Documentation

## Testing

There is a set of Mocha tests for the extension and extension localization which can be run with `npm test` and `npm run test-localization` or by `Launch Extension Tests` and `Launch Localization Tests`. Also there are e2e smoke tests placed in [`test/smoke`](https://github.com/microsoft/vscode-react-native/tree/master/test/smoke) folder that can be launched by `yarn smoke-tests` command or by `Launch All Smoke Tests` command. Make sure to [prepare test environment](https://github.com/microsoft/vscode-react-native/blob/master/test/smoke/docs/run-locally.md) before launching e2e tests.

Proposed Changes

Update CONTRIBUTING.md to include:

1. Test Types:

   • Unit tests: What they cover, how to run
   • Smoke tests: What they cover, prerequisites
   • Localization tests: Purpose and execution

2. Test Commands:

   # Run unit tests
   npm test
   
   # Run tests with coverage report
   gulp test:coverage
   
   # Run tests without rebuild
   gulp test-no-build
   
   # Watch mode for development
   gulp watch-build-test
   
   # Run localization tests
   npm run test-localization
   
   # Run smoke tests (requires environment setup)
   npm run smoke-tests
   

3. VS Code Launch Configurations:

   • List available launch configs
   • Explain when to use each one

4. Smoke Test Setup:

   • Summarize key prerequisites
   • Link to detailed setup guide
   • Mention platform-specific requirements

5. Test Coverage:

   • How to view coverage reports
   • Where coverage files are generated
   • Target coverage goals (if any)

Example Section

## Testing

The project has three types of automated tests:

### Unit Tests
Unit tests cover extension functionality, commands, debugger logic, and utilities.

**Run all unit tests:**
```bash
npm test

Run with coverage report:

gulp test:coverage

Skip rebuild (when code hasn't changed):

gulp test-no-build

VS Code launch config: Use Launch Extension Tests from the Debug panel.

Smoke Tests (E2E)

... [continue with smoke test details]

Localization Tests

... [continue with localization test details]

## Benefits

- Accurate documentation for contributors
- Clearer test execution workflow
- Better onboarding for new contributors
- Reduced confusion about test commands
- Alignment with actual project configuration

## References

- CONTRIBUTING.md line 30-31
- package.json test scripts
- gulpfile.js test tasks
- .vscode/launch.json configurations

Start with CONTRIBUTING.md and compare its Testing section against the test scripts in package.json, tasks in gulpfile.js, .vscode/launch.json, and test/smoke/docs/run-locally.md. Update the documentation to cover the requested test types, commands, launch configurations, prerequisites, and coverage details, with all commands matching the current project configuration.