test: use notice log level to make easily filter out logs from PID1

test/testsuite-04.units/logs-filtering.service

@@ -3,3 +3,4 @@ Description=Log filtering unit
 
 [Service]
 ExecStart=sh -c 'while true; do echo "Logging from the service, and ~more~"; sleep .25; done'
+SyslogLevel=notice

test/units/testsuite-04.sh

@@ -199,7 +199,7 @@ function run_service_and_fetch_logs() {
     journalctl --sync
     END=$(date '+%Y-%m-%d %T.%6N')
 
-    journalctl -q -u "$UNIT" -S "$START" -U "$END" | grep -Pv "systemd\[[0-9]+\]"
+    journalctl -q -u "$UNIT" -S "$START" -U "$END" -p notice
     systemctl stop "$UNIT"
 }