test: add UI test target for VM Wizard

Add new UI test infrastructure to validate wizard functionality:
- Create UTMUITests target
- Add VMWizardViewTest class with window close button test
- Add comprehensive TESTING.md guide covering:
  - Running tests via Xcode and command line
  - Test structure and organization
  - Guidelines for extending the test suite
  - Best practices for test naming

Author: alb3rto