test: speed up pytest runs with xdist

[scotluns]

Why

This reduces test runtime by letting pytest run safely in parallel. The STIX cache change is mostly there to make xdist practical: it avoids each worker doing the same setup work and keeps the parallel run from fighting over shared test data.

What changed

• Added pytest-xdist and switched CI to run pytest with two workers.
• Changed the ATT&CK STIX test fixture from an autouse session fixture to an explicit shared fixture, so tests only prepare STIX data when they need it.
• Store downloaded STIX bundles in .pytest_cache/attack-stix instead of a per-run temp directory.
• Warm the shared STIX cache before xdist workers start. Workers read from that cache instead of downloading or preparing bundles independently.
• Kept the STIX pytest options under tests/conftest.py, so --stix-enterprise, --stix-mobile, --stix-ics, --attack-version, and --stix-version still work with the xdist cache path.
• Removed the root-level conftest.py added on main; pytest setup now stays under tests/.
• Added just recipes and contributor guidance for parallel tests and STIX-backed test data.
• Added PyPI as the explicit uv index so lock generation is stable.

How to verify

• uv lock --check
• uv run ruff format --check tests/conftest.py tests/changelog/conftest.py
• uv run ruff check tests/conftest.py tests/changelog/conftest.py
• uv run --extra dev pytest --help
• uv run --extra dev pytest -n 2 --attack-version 19.0 tests/test_to_excel.py::test_enterprise_latest -q
• uv run --extra dev pytest -n 2 -q

Known gaps

No known follow-up work from local validation.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud