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

pytest.ini

@@ -0,0 +1,46 @@
+[pytest]
+# Pytest configuration for Masto-RSS
+
+# Test discovery patterns
+python_files = test_*.py
+python_classes = Test*
+python_functions = test_*
+
+# Output options
+addopts =
+    -v
+    --strict-markers
+    --tb=short
+    --cov=bot
+    --cov=main
+    --cov-report=term-missing
+    --cov-report=html
+    --cov-report=xml
+    --cov-fail-under=80
+
+# Test paths
+testpaths = .
+
+# Markers
+markers =
+    unit: Unit tests that don't require external services
+    integration: Integration tests that may use mocked external services
+    slow: Tests that take a long time to run
+
+# Coverage options
+[coverage:run]
+source = .
+omit =
+    test_*.py
+    .venv/*
+    */__pycache__/*
+    */site-packages/*
+
+[coverage:report]
+exclude_lines =
+    pragma: no cover
+    def __repr__
+    raise AssertionError
+    raise NotImplementedError
+    if __name__ == .__main__.:
+    if TYPE_CHECKING: