Bring tests against harbor back

In the commit https://github.com/carvel-dev/imgpkg/commit/eeb0b943eadc20d540912c7098cba00c16977d35 we removed the tests against harbor since the instance we were using no longer is available, but @vrabbi suggested looking into the possibility of using https://container-registry.com/ as an alternative.

This issue is to revert the above commit, but instead of using the registry, try out the other option

Inspect commit eeb0b943eadc20d540912c7098cba00c16977d35 to identify the removed Harbor tests and their entry points. Determine which alternative registry option is intended, then run the existing registry test suite against it. Done means the tests are restored and pass without relying on the unavailable Harbor instance.

Written by the indexing model from the issue text.