[test] Manage node connections better in mempool persist test

Summary:
there were two calls to disconnect_nodes that were no-ops. fixed one & removed
the other & added assertions to confirm node has no connections when creating
the unbroadcast transaction.

This is a backport of Core PR18807
https://github.com/bitcoin/bitcoin/pull/18807/commits/fa32e676e5833a5c5fc735ef00c0a80f5fab7a2c

Test Plan: test/functional/test_runner.py mempool_persist

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9033