Merge #14964: test: Fix race in mempool_accept

Summary:
PR14964 backport

Pull request description:

If we happen to pick the same random coin to spend, there would be mempool conflicts in some runs of the test. Fix that by popping from a static list of coins to spend from.

Depends on D5189

Test Plan: ninja check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5190