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, which lets local and CI runs reuse the same test data.
• Warm the shared STIX cache before xdist workers start. Workers then read from that cache instead of trying to download or prepare bundles independently.
• Fail clearly if an xdist worker needs a STIX bundle that was not prepared up front.
• Added just recipes for parallel test runs and coverage runs.
• 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 --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