ci: run pytest in parallel via -j 4

GitHub-hosted ubuntu-latest runners have 4 vCPUs, and the new test
harness supports pytest-xdist parallelism via run_tests.sh -j N.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: tridge

.github/workflows/ci.yml

@@ -27,7 +27,7 @@ jobs:
       run: |
         set -e  # Exit on any error
         echo "Running test script..."
-        scripts/run_tests.sh
+        scripts/run_tests.sh -j 4
         echo "All tests completed successfully"
 
   docker-test: