test: properly preserve journal from sd-bsod tests

test: properly preserve journal from sd-bsod tests

I (incorrectly) assumed that --relinquish-var does everything --flush
does, including moving already existing stuff from /var/log/journal/ to
/run/log/journal/, but that's not the case. To actually do that we need
to shuffle things manually, so let's do just that.

This should make issues like #31334 easier to debug, since with this
patch we now have a coredump in the test journal as well:

~# make -C test/TEST-04-JOURNAL/ clean setup run TEST_MATCH_SUBTEST=bsod BUILD_DIR=$PWD/build TEST_NO_NSPAWN=1
...
[   12.176089] testsuite-04.sh[712]: + echo 'Subtest /usr/lib/systemd/tests/testdata/units/testsuite-04.bsod.sh failed'
[   12.176089] testsuite-04.sh[712]: Subtest /usr/lib/systemd/tests/testdata/units/testsuite-04.bsod.sh failed
[   12.176089] testsuite-04.sh[712]: + return 1
[   12.177347] systemd[1]: testsuite-04.service: Failed with result 'exit-code'.
[   12.220580] systemd[1]: Failed to start testsuite-04.service.
Spawning getter /home/mrc0mmand/repos/@systemd/systemd/build/journalctl -o export -D /var/tmp/systemd-tests/systemd-test.Qtqmmr/root/var/log/journal...
Finishing after writing 7649 entries
TEST-04-JOURNAL: (failed; see logs)
-rw-r----- 1 root root 16777216 Feb 15 21:13 /var/tmp/systemd-tests/systemd-test.Qtqmmr/system.journal
...

~# coredumpctl --file /var/tmp/systemd-tests/systemd-test.Qtqmmr/system.journal
TIME                        PID UID GID SIG     COREFILE EXE                           SIZE
Thu 2024-02-15 21:13:38 CET 812   0   0 SIGABRT journal  /usr/lib/systemd/systemd-bsod    -