smokey: Close tmp file at end of sigdebug test

The rtdm test relies on successively free file descriptors which is
violated by this test when the run after each other.
Reported-by: Gylstorff Quirin <quirin.gylstorff@siemens.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

testsuite/smokey/sigdebug/sigdebug.c

@@ -292,6 +292,7 @@ static int run_sigdebug(struct smokey_test *t, int argc, char *const argv[])
 	err = sem_destroy(&send_signal);
 	check_no_error("sem_destroy", err);
 
+	close(tmp_fd);
 	if (wd) {
 		fprintf(wd, "%d", wdog_delay);
 		fclose(wd);