Consolidate usage of odb_at in tests by moving it to gix_testtools

[Christoph Rüßler (cruessler)]

This is a question based on my own experience working on adding SHA-256 support to tests as well as on this suggestion by an LLM: #2598 (review). Does it make sense to move odb_at into gix_testtools, in order to have a single shared implementation?

[Sebastian Thiel (Byron)]

Sorry, I don't know what this is about. Let's figure this out in chat :).