Find a "definitive" structure

We had a few back and forths with test structures, spec names, etc.
See https://github.com/ipfs/gateway-conformance/pull/79/files, https://github.com/ipfs/gateway-conformance/pull/92#discussion_r1245540339, and https://github.com/ipfs/gateway-conformance/pull/87#discussion_r1241958242.

Could we find a "definitive" structure for organizing tests and specs?
What about we use spec id or PR to the spec repo as a test name and spec id work?

Take cors, we could have `cors-000` (before specification, tests coming from sharness), then `cors-pr-423`, etc.

The current "mainstream" version of cors would be a spec group with all the specs that compose a regular cors gateway.
We could even pin these on dates, like `cors-2023` would be all the specs active during 2023.

End goal is:

- structure is deterministic so that contributors & users know how to go from spec to tests,
- this opens up a way to connect the process of adding test with adding IPIP specs.