tests/closure: Send SIGUSR1 only to our pid

Otherwise in e.g. the gnome-ostree integrationtest system, we
end up sending SIGUSR1 to the *entire session*, which triggers
various badness in untested debugging paths from gnome-session.

gobject/tests/closure.c

@@ -165,7 +165,7 @@ test_closure_fd (void)
 static gboolean
 send_usr1 (gpointer user_data)
 {
-  kill (0, SIGUSR1);
+  kill (getpid (), SIGUSR1);
   return FALSE;
 }