Do not test ETAG values

Brought up by @lidel and @hacdias on Slack:

https://filecoinproject.slack.com/archives/C04M8232QRW/p1685573605787349?thread_ts=1685531506.240809&cid=C04M8232QRW

> For the Etags specifically , the best IMO would be to move those tests to Boxo and remove them from both Kubo and the conformance. I think that’s something we should agree on.

> yeah, conformance is expecting boxo/kubo-specific Etag values ([example](https://github.com/ipfs/gateway-conformance/blob/e813393211643a905d3a62e7c547bc6c1e457028/tests/t0116_gateway_cache_test.go#L35)) because MVP was to port sharness.
> next step is to make these tests vendor-agnostic:
> Conformance should not care what is inside of Etag, but must test Etag is present, and unique to specific resource+response type.
Boxo/Kubo can test specific values.