Add tests

[onedr0p]

Not sure the best way to handle this but it maybe we could get contributions from the community and build a catalog of set commands to vyaml config file(s) and make sure they pass? Trying to cover all options in Vyos might not be easily achievable but if we cover enough uses cases it will give some confidence updates won't break.

[p3lim]

Functional testing could be something as simple as having a yaml file that covers a large section of the available options in yaml form, and a pre-tested commands file, then run the cli and see if it matches.

Unit testing is probably out of the question.