[test] Add code coverage framework

Implement a comprehensive test coverage framework that integrates with pytest-cov
to generate code coverage reports for the yt-dlp codebase. Key components:

- Configuration file (.coveragerc) with appropriate include/exclude patterns
- Helper script (run_coverage.py) with parallel report generation
- GitHub Actions workflow for automatic coverage reporting on PRs and pushes
- Support for Hatch testing environment and CLI integration
- Testing documentation for running coverage reports
- Sample test for demonstrating coverage reporting

test/README.md

@@ -0,0 +1,39 @@
+# yt-dlp Tests
+
+This directory contains tests for the yt-dlp codebase.
+
+## Running Tests
+
+### Using hatch (requires `pip install hatch`)
+
+```bash
+# Run tests for a specific test file
+hatch run hatch-test:run test/test_utils.py
+
+# Run a specific test class or method
+hatch run hatch-test:run test/test_utils.py::TestUtil
+hatch run hatch-test:run test/test_utils.py::TestUtil::test_url_basename
+
+# Run with verbosity
+hatch run hatch-test:run -- test/test_utils.py -v
+```
+
+### Using pytest directly
+
+```bash
+# Run a specific test file
+python -m pytest test/test_utils.py
+
+# Run a specific test class or method
+python -m pytest test/test_utils.py::TestUtil
+python -m pytest test/test_utils.py::TestUtil::test_url_basename
+
+# Run with verbosity
+python -m pytest -v test/test_utils.py
+```
+
+**Important:** Always run tests from the project root directory, not from a subdirectory.
+
+## Code Coverage
+
+For information on running tests with code coverage, see the documentation in `.coverage-reports/README.md`.