wallet: shuffle coins before grouping, where warranted

Summary:
Issue brought up in https://github.com/bitcoin/bitcoin/pull/12257\#discussion_r204554549

This is a bacport of Core PR13808

Test Plan:

make check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3414