test: sync journal after the test unit finishes

test: sync journal after the test unit finishes

In these two cases we need to sync the journal _after_ the unit finishes
as well, because we try to match messages from systemd itself, not
(only) from the unit, and the messages about units are dispatched
asynchronously.

That is, in the first case (silent-success.service) we want to make sure
that LogLevelMax= filters out messages _about_ units (from systemd) as
well, including messages like "Deactivated..."  and "Finished...", which
are sent out only when/after the unit is stopped.

In the second case we try to match messages with the "systemd" syslog
tag, but these messages come from systemd (obviously) and are sent out
asynchronously, which means they might not reach the journal before we
call `journalctl --sync` from the test unit itself, like happened here:

[ 1754.150391] TEST-04-JOURNAL.sh[13331]: + systemctl start verbose-success.service
[ 1754.172256] bash[13692]: success
[ 1754.221210] TEST-04-JOURNAL.sh[13694]: ++ journalctl -b -q -u verbose-success.service -t systemd
[ 1754.221493] TEST-04-JOURNAL.sh[13331]: + [[ -n '' ]]
[ 1754.175709] systemd[1]: Starting verbose-success.service - Verbose successful service...
[ 1754.221697] TEST-04-JOURNAL.sh[122]: + echo 'Subtest /usr/lib/systemd/tests/testdata/units/TEST-04-JOURNAL.journal.sh failed'
[ 1754.221697] TEST-04-JOURNAL.sh[122]: Subtest /usr/lib/systemd/tests/testdata/units/TEST-04-JOURNAL.journal.sh failed
[ 1754.221697] TEST-04-JOURNAL.sh[122]: + return 1
[ 1754.205408] systemd[1]: verbose-success.service: Deactivated successfully.
[ 1754.205687] systemd[1]: Finished verbose-success.service - Verbose successful service.

By syncing the journal after the unit is stopped we have much bigger
chance that the systemd messages already reached the journal - the race
is technically still there, but the chance we'd hit it should be pretty
negligible.

Resolves: #39555