test: use SYSLOG_IDENTIFIER= filter instead of "journalctl -u"

test: use SYSLOG_IDENTIFIER= filter instead of "journalctl -u"

"journalctl -u foo.service" may not work as expected, especially entries
for _TRANSPORT=stdout, for short-living services or when the service manager
generates debugging logs. Instead, SYSLOG_IDENTIFIER= should be reliable for
stdout. Let's use it.

An example case:
```
__CURSOR=s=06278e3bf011458e973c81d370a8f7a5;i=1e4dc;b=1b0258a5c78341609bf462c72d4541c3;m=308de65;t=6194c3895a13f;x=50c7e9af5b8cfc37
__REALTIME_TIMESTAMP=1716665017803071
__MONOTONIC_TIMESTAMP=50912869
_BOOT_ID=1b0258a5c78341609bf462c72d4541c3
SYSLOG_FACILITY=3
_UID=0
_GID=0
_MACHINE_ID=d3490e076ab24968bfa19a6aab26beb3
_HOSTNAME=H
_RUNTIME_SCOPE=system
_TRANSPORT=stdout
PRIORITY=6
_PID=2668
_STREAM_ID=3f9b8855636041988d003a9c63379b8a
SYSLOG_IDENTIFIER=echo
MESSAGE=foo
```
As you can see, there is no unit identifier.