parse: fix parsing of single number versions
parse.RE_NVR regex was incorrectly requiring at least one dot in version strings.
Fix it to support single number versions such as 1 or 20230101.
Also add a simple unit test for parse.split_archive_fn() that can be extended with supported version strings in the future to make sure versions can always be parsed from archive file names.
Fixes: #96 (closed)