Bolt 4: clarify onion test payload contents

[ellemouton]

This PR adds a clarifying comment to the onion-test.json file. The previous multi-frame test did not have lengths of payloads included in the hop payload field and instead had a separate field defining the payload type. This meant that the variable lengths still had to be added for the tlv payloads. However, in this new test - all the hops use the TLV type and so the payloads in this test do include the encoded lengths. This PR just adds a comment to clarify this.

[ellemouton]

sorry if I am completely wrong about this - It is just that I could only get things matching if I explicitly set the type to TLV. And things matched the current test vector if I set the type to legacy. So thought maybe it incorrect. But maybe Im also doing something wrong on my side

[t-bast]

I think I understand what's happening. The difference between your onion and the spec one is that you are pre-pending each hop.payload with its length, but they already contain their length.

For example, the first hop payload in the test vector is 12 02023a98 040205dc 06080000000000000001 where 12 is the length of the following tlv stream. In your onion, the first hop payload is 13 12 02023a98 040205dc 06080000000000000001.

The test case comment could be updated to make this explicit, this is really hard to figure out.

[ellemouton]

ah! ok that makes way more sense! Thanks for the help here @t-bast 🙏

I can update this PR so that it adds a clarifying comment to the test

[t-bast]

Thanks!