Add comprehensive test suite with GitHub Actions integration

[Copilot]

Created test infrastructure achieving 86% coverage with matrix testing across Python 3.9-3.12 and Ubuntu/Windows/macOS. No Codecov dependency.

Test Suite (23 tests)

• tests/test_avatars.py - Main avatars function, window creation, webbrowser integration
• tests/test_images.py, tests/test_links.py - Module validation
• tests/test_integration.py - Full application integration tests
• tests/test_main_pytest.py - Pytest-style tests with fixtures
• tests/conftest.py - Tkinter mocking for headless CI/CD environments

GitHub Actions Enhancement

Enhanced .github/workflows/pytest.yml:

• Matrix testing: Python 3.9-3.12 × Ubuntu/Windows/macOS (12 combinations)
• Coverage artifacts with HTML/XML reports
• Coverage summary in workflow output
• Explicit permissions (contents: read)

Infrastructure

• pytest.ini - Coverage configuration and test markers
• requirements.txt - Added pytest-cov, pytest-mock
• run_tests.{sh,bat} - Cross-platform test runners
• docs/TESTING.md - Testing documentation

Example Usage

# Run full suite with coverage
./run_tests.sh

# Or directly with pytest
pytest --cov=. --cov-report=html

Mocked tkinter allows tests to run in headless environments without display server dependencies.

Original prompt

Create a comprehensive test suite with a GitHub Actions integration. DO NOT use Codecov.

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💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

[github-actions]

Thank you for your contribution, please read and sign our CLA. Instructions on how to do so can be found in the CONTRIBUTING.md document.

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I have read the CLA Document, and I hereby sign the CLA.

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_{You can retrigger this bot by commenting recheck in this Pull Request. Posted by the CLA Assistant Lite bot.}

[willtheorangeguy]

Merging, even though some tests may fail.