Improve subctl unit tests

[dfarrell07]

Now that we have SonarCloud to better see coverage, it's clear that subctl is the main gap. We currently ahve 4.6% coverage.

https://sonarcloud.io/component_measures?id=submariner-io_subctl&metric=new_coverage&view=list

Add unit tests to get coverage up, maybe targeting 70ish%. This can be split into smaller issues, maybe this should be treated as an epic.

[stale]

This issue has been automatically marked as stale because it has not had activity for 60 days. It will be closed if no further activity occurs. Please make a comment if this issue/pr is still valid. Thank you for your contributions.

[stale]

This issue has been automatically marked as stale because it has not had activity for 60 days. It will be closed if no further activity occurs. Please make a comment if this issue/pr is still valid. Thank you for your contributions.

[tpantelis]

Duplicate of #422