Run tests/test_sample_quests.py serially on macOS CI

[Copilot]

tests/test_sample_quests.py::test_sample_quests crashes on macOS runners: the script it spawns performs native Graphviz/rendering work, which is unsafe when multiple pytest-xdist workers hit it concurrently. Rather than serializing the whole suite on macOS, only this test is taken out of the parallel run.

.github/workflows/tests.yml

• Split the single Run tests step into a Linux step (unchanged, -n 16 over the full suite) and a macOS step.
• macOS runs the suite in parallel with --ignore=tests/test_sample_quests.py and --cov-report= (data collected, no report), then runs that one test serially with --cov-append, emitting the report and enforcing --cov-fail-under=85 on the combined data.

- name: Run tests (MacOS)
  if: startsWith(matrix.os, 'macos')
  run: |
      pytest -n 16 -sv --cov=textworld --cov-report= --ignore=tests/test_sample_quests.py tests/ textworld/
      pytest -sv --cov=textworld --cov-append --cov-report=term-missing --cov-fail-under=85 tests/test_sample_quests.py

tests/test_sample_quests.py

• Spawn sys.executable instead of a bare python, so the subprocess uses the same interpreter as the test process rather than whatever python resolves to on the runner. Assertions unchanged.

Note the ordering: the serial invocation must come last since it owns --cov-append and the threshold check. Runners are ephemeral and no coverage cache is configured, so there is no stale .coverage to append to.

[azure-pipelines]

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