test: Synchronize journal before reading from it

There's a race condition in the sysuser test where it may try to read
entries from the journal before they are available. Fix it by adding a
`journalctl --sync` call.

BugLink: https://bugs.launchpad.net/bugs/1776654

test/TEST-21-SYSUSERS/test.sh

@@ -114,6 +114,7 @@ test_run() {
         prepare_testdir ${f%.input}
         cp $f $TESTDIR/usr/lib/sysusers.d/test.conf
         systemd-sysusers --root=$TESTDIR 2> /dev/null
+        journalctl --sync
         journalctl -t systemd-sysusers -o cat | tail -n1 > $TESTDIR/tmp/err
         if ! diff -u $TESTDIR/tmp/err  ${f%.*}.expected-err; then
             echo "**** Unexpected error output for $f"