3dbfb48946b9d2a98acef23674617510cf1b3386 tests: restore scalar_mul test (Jonas Nick)
Pull request description:
Without this commit, the res[i][1] test vectors are unused. They were introduced to test the correctness of scalar_sqr(x) and scalar_mul(x, x). These tests were deleted as part of removing scalar_sqr in commit
[5437e7bdfbffddf69fdf7b4af7e997c78f5dafbf](5437e7bdfb (diff-c2d5f1f7616875ab71cd41b053cfb428696988ff89642b931a0963d50f34f7e8L2195)).
Discovered in https://github.com/bitcoin-core/secp256k1/discussions/1463 by Coding-Enthusiast (thanks!).
ACKs for top commit:
real-or-random:
utACK 3dbfb48946b9d2a98acef23674617510cf1b3386
Tree-SHA512: 914e08db3efaa1cef546a9730096e740478c422d41fedb2b71ec3a7ea962f81740a05dc7e7c1fb191088f6d38b5690479c7d0864ca8abf2b2e9c4334f03ca605