Run Slurm script unit tests in CI

[fabrizio2210]

Summary

• add a test-python Makefile target using Python unittest discovery
• discover *_test.py suites in all the repo
• add a dedicated, change-gated GitHub Actions job that runs the target
• include the job in the existing ci-success dependency set
• skip cleanly only when neither test root contains tests

Why

This extracts the reusable unit-test runner from #2719 into a standalone change based on main. Keeping the runner independent lets Python tests in #2719, #2742, #2743, and future pull requests use the same CI job without depending on a health-check behavior change.

The discovery root cover all the Python scripts. In particular, this ensures #2743 runs its check_runner_test.py and write_soperator_metadata_test.py suites, as well as the existing worker tests.

PR #2743 is stacked on this branch so its checked-out head contains the Make target; #2742 remains stacked on #2743. Because the repository uses pull_request_target, the new job becomes available after this PR merges into main. Synchronizing #2743 after that merge will trigger the 109-test suite.

Validation

• env PYTHONDONTWRITEBYTECODE=1 make test-python on this branch: 102 tests passed
• the same target on the stacked [Superseded by #2774] Expose node RealMemory without controller RPC #2743 branch: 109 tests passed (4 Slurm-script tests and 105 worker tests)
• parsed .github/workflows/one_job.yml with PyYAML
• git diff --check origin/main...HEAD