rstest: add -f option.
rstest: add -f option.

when that option is set and
* -c is present, the hash file is overwritten
* no option is set, the sample is decoded, hash string is calculated
  and the hash file
  * exists         - then it is checked
  * does not exist - nothing happens, and the hash file is not created

TLDR: it -f does everything like no option, except all the
operations with the hash file

LebedevRI/rawspeed,darktable-org/rawspeed,LebedevRI/rawspeed,darktable-org/rawspeed,darktable-org/rawspeed,LebedevRI/rawspeed,darktable-org/rawspeed,LebedevRI/rawspeed,LebedevRI/rawspeed,darktable-org/rawspeed