[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

.github/workflows/coverage.yml

@@ -0,0 +1,66 @@
+name: Code Coverage
+
+on:
+  push:
+    branches: [ master ]
+  pull_request:
+    branches: [ master ]
+  workflow_dispatch:
+
+jobs:
+  coverage:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: '3.11'
+          cache: pip
+
+      - name: Install dependencies
+        run: |
+          python -m pip install --upgrade pip
+          pip install -e ".[dev]"
+          pip install pytest-cov
+
+      - name: Run coverage tests in parallel
+        run: |
+          # Create a simple script to run coverage tests in parallel
+          cat > run_parallel_coverage.py << 'EOF'
+          import concurrent.futures
+          import subprocess
+          import sys
+
+          def run_coverage(args):
+              test_path, module_path = args
+              cmd = ['python', '-m', 'devscripts.run_coverage', test_path, module_path]
+              return subprocess.run(cmd, check=True)
+
+          coverage_tests = [
+              ('test/test_utils.py', 'yt_dlp.utils'),
+              ('test/test_YoutubeDL.py', 'yt_dlp.YoutubeDL'),
+              ('test/devscripts', 'devscripts')
+          ]
+
+          with concurrent.futures.ThreadPoolExecutor(max_workers=3) as executor:
+              futures = [executor.submit(run_coverage, test) for test in coverage_tests]
+              for future in concurrent.futures.as_completed(futures):
+                  try:
+                      future.result()
+                  except subprocess.CalledProcessError as e:
+                      print(f"Error running coverage: {e}")
+                      sys.exit(1)
+          EOF
+
+          # Run the script
+          python run_parallel_coverage.py
+
+      - name: Archive coverage results
+        uses: actions/upload-artifact@v4
+        with:
+          name: coverage-report
+          path: |
+            .coverage-reports/html/
+            .coverage-reports/coverage.xml