Add practical examples to testing documentation

[Folyd]

Enhance the testing documentation with more practical examples and tutorials.

Tasks

• Add "Testing Recipes" section with common scenarios:
  • Testing async message flow
  • Testing with multiple parties
  • Testing error conditions
  • Testing with different transports
  • Testing VID operations
• Add video tutorial or animated GIFs showing:
  • Running tests locally
  • Interpreting test output
  • Debugging failing tests
  • Running coverage analysis
• Add troubleshooting guide with common issues
• Add FAQ section for testing
• Create "Testing Checklist" for contributors
• Add examples of good vs bad tests

Acceptance Criteria

• Documentation includes 10+ practical examples
• Common issues have troubleshooting steps
• Contributors report testing is easier to understand
• Testing checklist is followed in PRs

Priority

Low - Documentation improvement

Part of #245