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.
-------

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.

1 files changed, 5 insertions, 2 deletions

diff --git a/src/journal/test-journal-flush.c b/src/journal/test-journal-flush.c
index 50500222ad..5fedd3ed20 100644
--- a/src/journal/test-journal-flush.c
+++ b/src/journal/test-journal-flush.c
@@ -42,13 +42,16 @@ int main(int argc, char *argv[]) {
                 assert_se(f && f->current_offset > 0);
 
                 r = journal_file_move_to_object(f, OBJECT_ENTRY, f->current_offset, &o);
+                if (r < 0)
+                        log_error_errno(r, "journal_file_move_to_object failed: %m");
                 assert_se(r >= 0);
 
                 r = journal_file_copy_entry(f, new_journal, o, f->current_offset);
+                if (r < 0)
+                        log_error_errno(r, "journal_file_copy_entry failed: %m");
                 assert_se(r >= 0);
 
-                n++;
-                if (n > 10000)
+                if (++n >= 10000)
                         break;
         }