Replace the five string-matching tests from the salvaged commit with two
invariants that spawn a real child carrying a token in argv and check the
actual persistence boundaries: the /proc summary + warning line + JSON
snapshot, and the detached ps/pstree log written by the real writer (which
must also end up 0600 even when an earlier release created it 0644).
Drop the AttributeError guard around os.fchmod: spawn_async_diagnostic
returns None on win32 before reaching it, so the branch was unreachable.
c45ca8855d