flaky autopkgtest: SIGKILL is called but processes were already dead

See https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1077903

Copy paste:

From: Paul Gevers <elbrus@debian.org>
To: Debian Bug Tracking System <submit@bugs.debian.org>
Subject: debusine: flaky autopkgtest: SIGKILL is called but processes were already dead
Date: Sun, 4 Aug 2024 11:24:04 +0200

[Message part 1 (text/plain, inline)]

Source: debusine
Version: 0.4.1
Severity: serious
User: debian-ci@lists.debian.org
Usertags: flaky

Dear maintainer(s),

I looked at the results of the autopkgtest of your package, because the 
results were blocking the migration of src:autopkgtest. I noticed that 
it regularly fails.

Because the unstable-to-testing migration software now blocks on
regressions in testing, flaky tests, i.e. tests that flip between
passing and failing without changes to the list of installed packages,
are causing people unrelated to your package to spend time on these
tests.

Don't hesitate to reach out if you need help and some more information
from our infrastructure.

Paul

https://ci.debian.net/packages/d/debusine/testing/amd64/49938456/

 53s ======================================================================
 53s FAIL: test_execute_cmd_finished_before_killpg_sigkill 
(debusine.tasks.tests.test_task.RunCommandTaskTests.test_execute_cmd_finished_before_killpg_sigkill)
 53s Execute script. SIGKILL is called but processes were already dead.
 53s ----------------------------------------------------------------------
 53s Traceback (most recent call last):
 53s   File 
"/usr/lib/python3/dist-packages/debusine/tasks/tests/test_task.py", line 
1350, in test_execute_cmd_finished_before_killpg_sigkill
 53s     self.assertIn(
 53s AssertionError: 'output (contains stdout and stderr):\n\naborted: 
True\nreturncode: -9\n' not found in 'cmd: 
/tmp/debusine-tests-i0k_pvrb/signal-logger.sh 
/tmp/debusine-tests-i0k_pvrb 2091 2 TERM\noutput (contains stdout and 
stderr):\n\nFiles in working 
directory:\n2235.pid\n2236.pid\nsignal-logger.sh