Close subprocess STDOUT pipes in the test suite

[kurtmckee]

Spotted this while attempting to run the test suite locally with warnings enabled.

tests/test_cli.py::test_cli_fails_to_start_given_an_invalid_broker_name
tests/test_cli.py::test_cli_fails_to_start_given_an_invalid_broker_instance
tests/test_cli.py::test_cli_fails_to_start_given_an_invalid_nested_broker_instance
  /home/kurt/dev/pr-dramatiq/.venv/lib/python3.12/site-packages/_pytest/runner.py:141:
ResourceWarning: unclosed file <_io.FileIO name=12 mode='rb' closefd=True>

The three tests listed are the only ones that read from STDOUT pipes; closing the pipe in the pytest fixture is the most reliable way to address this.