test data corpus. Updated filesystem store tests accordingly:
- Remove comments from all_versions tests stating that multiple
versions are not supported. Improve the tests to ensure that
all versions are in fact retrieved.
- Update the get() test to assure that it gets only the latest
version, when there is more than one version.
- Update some count checks, since there are more objects now
- Fix some typos
Factored out the _is_marking() function from the memory datastore
module to utils so it can be reused, and changed both filesystem
and memory datastore modules to import and use it.