Fix file descriptor leak in hstox test.

We return E_WRITE because closing only fails when an I/O error occurs,
which is likely an error from the write() call above. See close(2) for
details.

http://man7.org/linux/man-pages/man2/close.2.html

testing/hstox/driver.c

@@ -89,6 +89,7 @@ static int write_sample_input(msgpack_object req)
     msgpack_pack_object(&pk, req);
 
     check_return(E_WRITE, write(fd, sbuf.data, sbuf.size));
+    check_return(E_WRITE, close(fd));
 
     return E_OK;
 }