tests: Initialize random group elements fully
Summary:
Also fix add a missing comment.
This is a backport of secp56k1 [[https://github.com/bitcoin-core/secp256k1/pull/814 | PR814]]
Test Plan:
ninja check-secp256k1
Reviewers: #bitcoin_abc, jasonbcox
Reviewed By: #bitcoin_abc, jasonbcox
Differential Revision: https://reviews.bitcoinabc.org/D7613