Convenience access for versions in checks

for a group of sum checks, expose a function that provides all the
tarsum.Version's present.
Tests for sum checks Versions
dockertarsum: cleanup the checks process

* Added helper functions for loading the check files easier
* validate whether the check files include the version of tarsum being
  tested for.
* put error output to stderr
dockertarsum: default to TarSum Version1

and clarify a couple of comments