Tests: Support HEAD requests in oci-registry-server

This just does a GET, which is not quite right, but will work.
This is needed for the authenticator.

(cherry picked from commit 530475b9ab)
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f