mitogen: _first_stage() EOF experiments

[moreati]

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[moreati]

Possible data point, possible noisy test. In https://github.com/mitogen-hq/mitogen/actions/runs/20068591776/job/57565192791 ` testing 1356c43 with timeout=10 one failure occured, only on u2204 py27-m_mtg

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FAIL: test_stage (first_stage_test.CommandLineTest)
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Traceback (most recent call last):
  File "/home/runner/work/mitogen/mitogen/tests/first_stage_test.py", line 78, in test_stage
    self.assertIn(b'shutting down', stdout)
AssertionError: 'shutting down' not found in 'MITO000\nMITO001\nMITO002\n'

In the next run (https://github.com/mitogen-hq/mitogen/actions/runs/20069213928/job/57567375542 testing 3ba087e), without a timeout it passed.