use FileFd in HashSum test to unbreak non-linux ports

use FileFd in HashSum test to unbreak non-linux ports

The testcode happily mixes FILE* operations and direct access to fds
which is even a bit suprising that it works on linux and worked so
long for non-linux ports, so we switch to usage of FileFd instead
which provides us with simple fd-only operations. Its overkill for this
test as its a bare file and we ask for the descriptor all the time, but
it shouldn't hurt to implicitly test it a bit this way.

Closes: 721723
Thanks: Aaron M. Ucko