Scoru,Test: test encoding of new internal messages

Context

Part of #4141 (closed)

The encoding needs to be mirrored in the kernels, we use these tests to produce the bytes produced by the encoding.

Manually testing the MR

CI should be enough.

Checklist

• n/a Document the interface of any function added or modified (see the coding guidelines)
• n/a Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• n/a For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR