Add test that invokes gz and expects exact string output

[scpeters]

Desired behavior

From #178 (comment):

it would be ideal to add a test to this repo that invokes gz and expects an exact output string, since that is the reason why sdformat's test caught this error message

I think we could add a test command and then invoke gz --commands and verify the exact text output. I'll create a feature request for this unless you feel moved to implement it right now

Alternatives considered

Implementation suggestion

Additional context