test: temporarily enable session lingering for the test user

test: temporarily enable session lingering for the test user

So the user daemon with the long-running service is not killed while we
test the journal:

[  834.077080] testsuite-04.sh[10937]: Running as unit: user-sleep.service; invocation ID: b1f2c9c9a14a40ce836c867139d14dc8
[  834.077687] systemd[10943]: Started user-sleep.service.
[  834.078437] (sd-pam)[10952]: pam_unix(login:session): session closed for user testuser
[  834.078643] systemd[1]: run-u3.service: Deactivated successfully.
[  834.078710] testsuite-04.sh[10930]: + for _ in {0..9}
[  834.078710] testsuite-04.sh[10930]: + journalctl --rotate
...
[  834.081253] systemd[1]: session-7.scope: Deactivated successfully.
...
[  844.602065] testsuite-04.sh[10930]: + journalctl --rotate
[  844.630414] testsuite-04.sh[10930]: + journalctl --sync
[  844.632005] systemd[1]: Stopping user@4711.service...
[  844.634179] systemd[10943]: Activating special unit exit.target...
[  844.635769] systemd[10943]: Stopped target default.target.
[  844.636136] systemd[10943]: Stopped target timers.target.
[  844.636479] systemd[10943]: Stopping user-sleep.service...
[  844.636998] systemd[10943]: Stopped user-sleep.service.
...
[  844.758893] testsuite-04.sh[10930]: + systemctl stop --user -M testuser@ user-sleep.service
[  845.213399] systemctl[11066]: Failed to stop user-sleep.service: Unit user-sleep.service not loaded.