test: Fix cleanup of lingering user systemd managers

Users who are not logged in are *not* showing up in `loginctl
list-users`, unlike I originally assumed. To make sure that no "systemd"
processes from users are left over (like `TestAccounts.testRootLogin`
does), just stop *all* `user@*.service` after each test. This has the
added benefit that it should always succeed (and if it doesn't, we
really want to know).

test/common/testlib.py

@@ -1585,7 +1585,6 @@ def terminate_sessions():
             # on OSTree we don't get "web console" sessions with the cockpit/ws container; just SSH; but also, some tests start
             # admin sessions without Cockpit
             self.machine.execute("""for u in $(loginctl --no-legend list-users  | awk '{ if ($2 != "root") print $1 }'); do
-                                        systemctl stop user@$u.service 2>/dev/null || true
                                         loginctl terminate-user $u 2>/dev/null || true
                                         loginctl kill-user $u 2>/dev/null || true
                                         pkill -9 -u $u || true
@@ -1613,6 +1612,9 @@ def terminate_sessions():
                     m.execute(f"loginctl session-status {s} >&2")
                     raise
 
+            # terminate all systemd user services for users who are not logged in
+            self.machine.execute("systemctl stop user@*.service")
+
         self.addCleanup(terminate_sessions)
 
     def tearDown(self):