Separate test runs for Unit versus Integration

[trexfeathers]

• Unit test run: runs using core dependencies only
  • Runs against the 3 SPEC-0 Python versions (so will need locked env specs for each)
• Integration test run: runs using core and optional dependencies
  • Runs against only the latest SPEC-0 Python version, to save resource (could we run against all 3 for the lock update PRs only?)
• Split should ideally be easy to run in CI AND locally (i.e. Nox or Pixi). At minimum the GHA CI should be split

(Need to think about testing built wheels too. Currently we just test import, and that is against all 3 Python versions)