Fix (inverse) meaning of -persistmempool

Summary: This is a backport of core#23061

Test Plan:
ninja all check-all

I checked that applying the test change without the code changes causes mempool_persist.py to fail.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11944