test-oci-registry.sh: Tests for talking to an OCI registry

Add a new test case to test the OCI remote functionality. The tests talk to
a server that implements good-enough index generation and bits of the
docker registry protocol. Adding and remove remotes, summary and appstream
generation, and image installation are all tested.

Closes: #1910
Approved by: alexlarsson
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f