Replace check sum tests for packing/unpacking
HltPackedDataDecoder/Writer had some checksum checks which now should be moved to packer/unpackers.
The test was disabled due to fluctuating checksums as reported LHCb#151 (closed)
Add the checks to packers and see if they still fluctuate.
This is a follow up to LHCb!3268 (merged)
Edited by Sevda Esen