core: don't enable special signals in test mode

Fixes:
$ systemd-analyze verify ...
Failed to open /dev/tty0: Permission denied

src/core/manager.c

@@ -380,6 +380,9 @@ static int enable_special_signals(Manager *m) {
 
         assert(m);
 
+        if (m->test_run)
+                return 0;
+
         /* Enable that we get SIGINT on control-alt-del. In containers
          * this will fail with EPERM (older) or EINVAL (newer), so
          * ignore that. */