regtest: Read test results in chunks to get the md5 digest
regtest: Read test results in chunks to get the md5 digest

Instead of creating the md5 object with the entire file, we feed it with
chunks of data using the update method. This improves a lot the memory
consumption and performance as well.

GNOME/libgxps,GNOME/libgxps,GNOME/libgxps