test: temporarily enable session lingering for the test user #2

Similarly to bbac11c we need to enable session lingering for the test
user, so the long-running test units are not killed prematurely:

[   18.822261] testsuite-55.sh[403]: + systemctl start --machine testuser@.host --user testsuite-55-testchill.service
[   18.852775] systemd[1]: Started run-u17.service.
[   19.256431] (o-bridge)[526]: pam_unix(login:session): session opened for user testuser(uid=4711) by testuser(uid=0)
[   19.288346] systemd[1]: Started session-2.scope.
[   20.165874] systemd[392]: Created slice session.slice.
[   20.166459] systemd[392]: Starting dbus-broker.service...
[   20.220189] dbus-broker-launch[529]: Policy to allow eavesdropping in /usr/share/dbus-1/session.conf +31: Eavesdropping is deprecated and ignored
[   20.220189] dbus-broker-launch[529]: Policy to allow eavesdropping in /usr/share/dbus-1/session.conf +33: Eavesdropping is deprecated and ignored
[   20.220494] systemd[392]: Started dbus-broker.service.
[   20.224276] dbus-broker-launch[529]: Ready
[   20.231702] systemd[392]: Created slice testsuite.slice.
[   20.231976] systemd[392]: Created slice testsuite-55.slice.
[   20.232259] systemd[392]: Created slice testsuite-55-workload.slice.
[   31.065294] testsuite-55.sh[403]: + systemctl start --machine testuser@.host --user testsuite-55-testbloat.service
[   31.065641] (sd-pam)[528]: pam_unix(login:session): session closed for user testuser
[   31.066103] (sd-pam)[528]: pam_systemd(login:session): Failed to release session: Access denied
[   31.066152] systemd[392]: Started testsuite-55-testchill.service.
[   31.068062] systemd[1]: run-u17.service: Deactivated successfully.
[   31.068217] dbus-broker[389]: A security policy denied :1.20 to send method call /org/freedesktop/login1:org.freedesktop.login1.Manager.ReleaseSession to org.freedesktop.login1.
[   31.075901] (o-bridge)[537]: pam_unix(login:session): session opened for user testuser(uid=4711) by testuser(uid=0)
[   31.091098] systemd[1]: Stopping session-2.scope...
[   31.092158] systemd[1]: Started run-u21.service.
[   31.092993] systemd[1]: session-2.scope: Deactivated successfully.
[   31.093287] systemd[1]: Stopped session-2.scope.
[   31.095798] systemd[1]: Stopping user@4711.service...
[   31.103541] systemd[392]: Activating special unit exit.target...
[   31.108359] systemd[392]: Stopped target default.target.
[   31.109798] systemd[392]: Stopped target timers.target.
[   31.110790] systemd[392]: Stopping testsuite-55-testchill.service...
[   31.112154] systemd[392]: Stopped testsuite-55-testchill.service.
[   31.114033] systemd[392]: Removed slice testsuite-55-workload.slice.
[   31.114971] systemd[392]: Removed slice testsuite-55.slice.
[   31.115858] systemd[392]: Removed slice testsuite.slice.
...
[   31.475949] testsuite-55.sh[403]: + systemctl --machine testuser@.host --user status testsuite-55-testchill.service
[   31.490464] systemd[1]: session-3.scope: Deactivated successfully.
[   31.565929] systemd[1]: Started run-u33.service.
[   31.592437] (o-bridge)[583]: pam_unix(login:session): session opened for user testuser(uid=4711) by testuser(uid=0)
[   31.610210] systemd[1]: Started session-5.scope.
[   31.616960] testsuite-55.sh[578]: ○ testsuite-55-testchill.service - No memory pressure
[   31.616960] testsuite-55.sh[578]:      Loaded: loaded (/usr/lib/systemd/tests/testdata/units/testsuite-55-testchill.service; static)
[   31.616960] testsuite-55.sh[578]:      Active: inactive (dead)
[   31.617438] (sd-pam)[586]: pam_unix(login:session): session closed for user testuser

Addresses systemd#31426 (comment).

test/units/testsuite-55.sh

@@ -117,6 +117,7 @@ if systemctl status testsuite-55-testbloat.service; then exit 42; fi
 if ! systemctl status testsuite-55-testchill.service; then exit 24; fi
 
 # Make sure we also work correctly on user units.
+loginctl enable-linger testuser
 
 systemctl start --machine "testuser@.host" --user testsuite-55-testchill.service
 systemctl start --machine "testuser@.host" --user testsuite-55-testbloat.service
@@ -144,6 +145,8 @@ done
 if systemctl --machine "testuser@.host" --user status testsuite-55-testbloat.service; then exit 42; fi
 if ! systemctl --machine "testuser@.host" --user status testsuite-55-testchill.service; then exit 24; fi
 
+loginctl disable-linger testuser
+
 # only run this portion of the test if we can set xattrs
 if cgroupfs_supports_user_xattrs; then
     sleep 120 # wait for systemd-oomd kill cool down and elevated memory pressure to come down