Increase test coverage

[davidcollom]

There is currently only 38% total test coverage, to help improve collaboration and ensure a stable experience we should look to increase this (target TBA) significantly.

This should also be enforced within GitHub Actions and can be updated here: https://github.com/jetstack/version-checker/blob/b27e0afd5d5cfc517d0139061fe5a105e8eee7b0/.testcoverage.yml

[github-actions]

This issue has been automatically marked as stale because it has not had
any activity in the last 90 days. It will be closed if no further activity
occurs. Thank you for your contributions.

[github-actions]

This issue has been automatically marked as stale because it has not had
any activity in the last 90 days. It will be closed if no further activity
occurs. Thank you for your contributions.