Add test vectors for BIP 341 SigMsg

[giacomocaironi]

Add some tests to Taproot SigMsg, inspired by BIP 143 test vector

[jonasnick]

Hey @giacomocaironi, thanks a lot for your work. This looks like it would be quite helpful to a BIP 341 implementer.

I think test vectors should be part of the reference code's test harness. Is it already? This would be the best way to ensure correctness of the test vectors. Also, I'm curious how you generated the vectors.

[maflcko]

Previous discussion thread at https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2021-September/019466.html

[giacomocaironi]

These test vectors are new, I created them from scratch them by randomly generating the keys. The reason I did this is that the official test vector contains non-standard transactions (they have strange version values). I thought it would by a nice addition if we added standard transactions to the test vectors, like BIP 143 did.

But if the hassle is too big, especially near the activation date, I can ignore this and simply pick some transactions from the official test vector instead of mine.

[MegoyTambayan]

Nicely Done!

[sipa]

@giacomocaironi I think @jonasnick is asking that these test vectors are added to Bitcoin Core's implementation, so that we can be sure they match the consensus implementation there.

I'll try implementing them.

[giacomocaironi]

Closing. This PR is superseded by #1225