oci/*: add tests to newly split out code

[cyphar]

After #90, it turns out that most packages didn't test half of their code directly -- meaning that after the split there are fewer packages with tests.

• Add tests for all of the new split components which are now missing tests (they didn't have unit tests previously).

[cyphar]

I believe this was done a very long time ago, though I didn't write enough information at the time for me to remember what this issue was really about. Whoops.