Fix silently-passing subprocess tests and process resource leak

[Copilot]

run_in_subprocess never checked p.exitcode, so subprocess crashes were invisible — tests reported PASSED while error tracebacks printed to stderr. This masked a pre-existing bug in test_coreclr_properties and left processes unclosed on failure.

Changes

• Assert subprocess exit code — run_in_subprocess now asserts p.exitcode == 0, turning silent subprocess failures into actual test failures.

• Wrap assertion in try/finally — ensures p.close() is always called, preventing ResourceWarning: process not closed when the assertion raises.

• Fix test_coreclr_properties kwarg mis-passing — was passing properties=dict(APP_CONTEXT_BASE_DIRECTORY=...), which **properties captured as {'properties': {'APP_CONTEXT_BASE_DIRECTORY': ...}}, causing AttributeError: 'dict' object has no attribute 'encode' in the subprocess. Fixed by passing the kwarg directly:

  # Before (broken)
  run_in_subprocess(..., properties=dict(APP_CONTEXT_BASE_DIRECTORY=str(example_netcore)))
  
  # After (correct)
  run_in_subprocess(..., APP_CONTEXT_BASE_DIRECTORY=str(example_netcore))

• Move fixtures to conftest.py — separates build fixtures from test logic.

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