test-journal-flush: provide at least *some* logs

Ubunut autopkgtest fails with:
405/501 test-journal-flush                      FAIL     0.74 s (killed by signal 6 SIGABRT)

--- command ---
SYSTEMD_KBD_MODEL_MAP='/tmp/autopkgtest.BgjJJv/build.yAM/systemd/src/locale/kbd-model-map' SYSTEMD_LANGUAGE_FALLBACK_MAP='/tmp/autopkgtest.BgjJJv/build.yAM/systemd/src/locale/language-fallback-map' PATH='/tmp/autopkgtest.BgjJJv/build.yAM/systemd/build-deb:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games' /tmp/autopkgtest.BgjJJv/build.yAM/systemd/build-deb/test-journal-flush
--- stderr ---
Assertion 'r >= 0' failed at src/journal/test-journal-flush.c:48, function main(). Aborting.
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It's hard to say what is going on here without any error messages whatsoever.
The test goes into deep details of journal file handling, so it needs to also
do logging on its own.