[backport#14047 2/4] QA: add test for CKey::Negate()

Summary:
https://github.com/bitcoin/bitcoin/pull/14047/commits/3b64f852e400c552f031697d6a86829dc6e74bd6

Depends on D7393

Partial backport of Core PR14047

Test Plan:

ninja check-bitcoin-key_tests

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7394