Add more IPNS record tests based on IPIP-428

[lidel]

Ref. ipfs/specs#428 tldr: we should have .ipns-record files with fixtures for:

• V1-only → record invalid (gateway returns 5XX error)
• V1+V2 (both signatures valid) → record valid, HTTP 200
• V1+V2 (both signatures valid, but 'value' is different in V1 pb) → record invalid (gateway returns 5XX error)
• V1+V2 (only signatureV1 valid) → record invalid (gateway returns 5XX error)
• V1+V2 (only signatureV2 valid) → record valid, HTTP 200
• V2-only (no V1 fields) → record valid, HTTP 200
• update IPIP-428 and include links to mentioned fixtures from this PR

This will be an extremely useful resource for anyone trying to implement IPNS, as they would be able to either run gateway-conformance, or at least reuse related fixtures.

[lidel]

This is blocking ipfs/specs#428 from being merged.
@hacdias would you be able to tackle this before diving into unixfs?
(since you've done GO and JS implementation of IPIP-428 and have the relevant context)