test: improve OCI unit testing

[AustinAbro321]

Description

This is work that I've broken out of #4843. This introduces a test fixture for unit testing OCI objects so our tests don't need to rely on the upstream registry

Checklist before merging

• Test, docs, adr added or updated as needed
• Contributor Guide Steps followed

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
see 8 files with indirect coverage changes

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[brandtkeller]

Minor go backs but it also prompted a thought; I wonder if we need v1 documentation regarding exported APIs that are not included under the project umbrella of supported API's. We should be able to make breaking changes to API's under testing utilities?

[AustinAbro321]

I would consider test APIs outside of the stable API promise

[brandtkeller]

lgtm