Run the tests that need no Elasticsearch host in GitHub Actions #44

The repository has no .github/workflows, so nothing runs the test suite on a pull request. Most of tests/ needs the su10:9200 Elasticsearch host (test_set_identifiers.py, test_normalized_nodes.py), but test_cors.py, test_health.py, test_version.py and test_openapi.py (added in #42) run anywhere, and that is enough to catch the kind of drift that has gone unnoticed: before #42, the POST set-ID test posted a single object where the handler expects a list, and had evidently not been run in some time.

Suggested first step: a workflow that installs the package with the tests extra and runs those four files on pushes and pull requests. Running the Elasticsearch-backed tests would need either a fixture index in a service container or a reachable test cluster, and is a second step; #19 may be the place that gets decided.

Blocked on #43 if the workflow uses uv run; uv pip install works today.