btcec/schnorr/musig2: add infinity testvectors
btcec/schnorr/musig2: add infinity testvectors

This commit adds the testvectors from
https://github.com/jonasnick/bips/commit/20ba03106d245d42375e65f739c58dd9074daf6e

tb00/btcd,btcsuite/btcd,btcsuite/btcd,tb00/btcd