tests: restore scalar_mul test #1474

1. 
   jonasnick commented at 3:52 pm on January 4, 2024: contributor

   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.

   Discovered in https://github.com/bitcoin-core/secp256k1/discussions/1463 by Coding-Enthusiast (thanks!).

2. tests: restore scalar_mul test

   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.

   3dbfb48946
3. real-or-random added the label assurance on Jan 4, 2024
4. real-or-random added the label refactor/smell on Jan 4, 2024
5. real-or-random approved
6. 
   real-or-random commented at 4:29 pm on January 4, 2024: contributor

   utACK 3dbfb48946b9d2a98acef23674617510cf1b3386
7. real-or-random merged this on Jan 4, 2024
8. real-or-random closed this on Jan 4, 2024

   ---

Contributors
jonasnick real-or-random

Labels
assurance refactor/smell