[backport#14931] test: mempool_persist: Verify prioritization is dumped correctly

Summary:
fa30a0e7f7 test: mempool_persist: Verify prioritization is dumped correctly (MarcoFalke)

https://github.com/bitcoin/bitcoin/pull/14931/commits/fa30a0e7f7bb3f93c15bc4d8922eec11b9c7d45f

Backport of Core PR14931

Test Plan:

test_runner.py mempool_persist

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6522