Improve coverage of zinoconv program

[lunkwill42]

Although the zinoconv program introduced in #174 is a one-off program for some very few users that have been using Zino 1, and doesn't need the same level of maintainability as the rest of Zino 2, it's lack of full test coverage reduces the existing good overall numbers of the Zino 2 codebase.

Some effort should be expended in increasing its test coverage, but it is not an urgent issue.