Add test vector for multiproof

[inahga]

A formal test vector for VDAFs using multiple proofs is necessary for ensuring interop between VDAF implementations. I generated this test vector from some modifications to the VDAF reference implementation.

I used the code point 0xFFFFFFFF since that's used elsewhere for test VDAFs. I suppose we can assign it some codepoint in 0xFFFF0000 - 0xFFFFFFFF, but it hasn't seemed necessary to give it a real-ish codepoint yet.

Note I moved each test into a mod tests module to resolve unused import warnings, consider viewing the diff without whitespace.