docs: Add comprehensive testing guide for zx scripts

[Maxiemad]

Fixes #1297

Summary

This PR adds comprehensive documentation and working examples for testing zx scripts, addressing the question "How do I write tests for zx scripts?" raised in issue #1297.

Changes

• ✅ Added complete testing documentation at docs/testing.md (499 lines)
• ✅ Added working example script at examples/testing-example.mjs
• ✅ Added complete test suite example at examples/testing-example.test.mjs (all 8 tests passing)
• ✅ Updated VitePress config to include testing guide in navigation

What's Included

The documentation covers multiple approaches to testing zx scripts:

1. Mocking $.spawn - For unit testing (as recommended by @antongolub in the issue)
2. Using custom spawn functions - For more control over command mocking
3. Extracting logic to testable functions - Refactoring approach
4. Integration testing - Testing with real commands
5. Context isolation with within() - Preventing test interference
6. Best practices - Patterns and helper functions

Usage Example

import { $, within } from 'zx'
import { createMockSpawn } from './test-helpers.mjs'

test('deploy function', async () => {
await within(async () => {
$.spawn = createMockSpawn({
'git branch --show-current': { stdout: 'main\n', exitCode: 0 },
})

const result = await deploy()
assert.equal(result.branch, 'main')

})
})## Verification

• ✅ All examples tested and verified to work
• ✅ Test suite passes all 8 tests
• ✅ Documentation builds correctly
• ✅ Code examples are correct and tested

Related

Addresses #1297 - "How do I write tests for zx scripts?"

---

• Build: I've run npm build before committing and verified the bundle updates correctly.
• Tests: I've run npm test and confirmed all tests succeed.
• Docs: I've added or updated relevant documentation as needed.
• CoC: My changes follow the project's coding guidelines and Code of Conduct.
• Review: This PR represents original work and is not solely generated by AI tools.

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