test: refactor: clarify the coins simulation

Summary:
Adds comments, slight refactor clarifications to make the code
easier to follow.

This is a partial backport of core#17487
https://github.com/bitcoin/bitcoin/pull/17487/commits/6d8affca96c7a34f5f104c5a3122e7420ffc083c

Depends on D16156

Test Plan: ninja check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16161