test: Set kernel loglevel to INFO when running tests unattended

This makes sure all kernel log messages are logged to the console.
This should be helpful during shutdown to detect possible issues with
journald when the logs can't be written to the journal itself anymore
but are written to kmsg.

test/integration-test-wrapper.py

@@ -373,6 +373,7 @@ def main() -> None:
                         'systemd.show_status=error',
                         'systemd.crash_shell=0',
                         'systemd.crash_action=poweroff',
+                        'loglevel=6',
                     ]
                     if not sys.stderr.isatty()
                     else []