t/helper/test-truncate: close file descriptor after truncation

Fix a resource leak where the file descriptor was not closed after
truncating a file in t/helper/test-truncate.c.

Signed-off-by: Hoyoung Lee <lhywkd22@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/helper/test-truncate.c

@@ -21,5 +21,8 @@ int cmd__truncate(int argc, const char **argv)
 
 	if (ftruncate(fd, (off_t) sz) < 0)
 		die_errno("failed to truncate file");
+
+	close(fd);
+
 	return 0;
 }