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