Add comprehensive test suite with GitHub Actions CI/CD

- Refactor code into testable bot.py module with MastodonRSSBot class
- Create 20+ unit tests covering core functionality and edge cases
- Create 10+ integration tests for RSS parsing and Mastodon posting
- Add GitHub Actions workflow for automated testing
  - Unit tests on Python 3.10, 3.11, 3.12
  - Integration tests with mocked external services
  - Code quality checks (flake8, black, mypy)
  - Docker build validation
- Configure pytest with 80% minimum coverage requirement
- Add test dependencies in requirements-test.txt
- Update .gitignore to exclude test artifacts
- Add comprehensive TESTING.md documentation
- Add test status badge to README
- Maintain full backward compatibility with existing setup

requirements-test.txt

@@ -0,0 +1,21 @@
+# Test dependencies for Masto-RSS bot
+# Install with: pip install -r requirements-test.txt
+
+# Base requirements
+-r requirements.txt
+
+# Testing framework
+pytest==7.4.4
+pytest-cov==4.1.0
+pytest-mock==3.12.0
+
+# HTTP mocking for integration tests
+responses==0.24.1
+
+# Code quality
+flake8==7.0.0
+black==24.1.1
+mypy==1.8.0
+
+# Coverage reporting
+coverage==7.4.0