Fix pytest deprecation warning

[mrcljx]

Running tests in verbose mode I noticed this warning:

t/unit/test_spawn.py::test_spawn::test_start
  site-packages/_pytest/python.py:170: PytestReturnNotNoneWarning: Test functions should return None, but t/unit/test_spawn.py::test_spawn::test_start returned <class 'int'>.
  Did you mean to use `assert` instead of `return`?
  See https://docs.pytest.org/en/stable/how-to/assert.html#return-not-none for more information.

Fixed it by replacing return with assert

[Copilot]

Pull request overview

This PR updates a unit test to comply with pytest’s requirement that tests return None, eliminating a deprecation warning by asserting on the process exit code instead of returning it.

Changes:

• Replaced return p.exitcode in test_start with an assertion that p.exitcode == 0 to align with pytest best practices and remove PytestReturnNotNoneWarning.

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