docs: Add comprehensive testing guide for zx scripts

- Add complete testing documentation at docs/testing.md
- Add working example script at examples/testing-example.mjs
- Add complete test suite example at examples/testing-example.test.mjs
- Update VitePress config to include testing guide in navigation

Fixes #1297

This PR addresses the issue asking for best practices on how to test zx scripts.
The documentation covers:
- Mocking $.spawn for unit testing
- Using custom spawn functions
- Extracting logic to testable functions
- Integration testing approaches
- Context isolation with within()
- Complete working examples

All examples have been tested and verified to work correctly.

Author: Maxiemad

docs/.vitepress/config.mts

@@ -67,6 +67,7 @@ export default defineConfig({
             { text: 'Configuration', link: '/configuration' },
             { text: 'Process Promise', link: '/process-promise' },
             { text: 'Process Output', link: '/process-output' },
+            { text: 'Testing', link: '/testing' },
             { text: 'Contribution Guide', link: '/contribution' },
             { text: 'Architecture', link: '/architecture' },
             { text: 'Migration from v7', link: '/migration-from-v7' },