test: Create InsecureRandMoneyAmount() helper, de-duplicate add_coin methods
Summary:
Generate semi-random Amounts up to MAX_MONEY rather than only uint32, and use it in the unit tests.
This concludes backport of core#26490
https://github.com/bitcoin/bitcoin/pull/26940/commits/9d92c3d7f42c18939a9a6aa1ee185f1c958360a0
https://github.com/bitcoin/bitcoin/pull/26940/commits/4275195606e6f42466d9a8ef766b3035833df4d5
Depends on D16155
Test Plan: ninja check
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Subscribers: Fabien
Differential Revision: https://reviews.bitcoinabc.org/D16156