[backport#19507] Have zmq reorg test cover mempool txns

Summary:
https://github.com/bitcoin/bitcoin/pull/19507/commits/7356292e1d7a44da8a2bd31c02c58d550bf38009

Depends on D7348

Concludes backport of Core PR19507

Test Plan:

test_runner.py interface_zmq

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7349