test: Fix intermittent error in mempool_reorg

Summary:

Also speed up tx relay while touching the file anyway.

This should fix the intermittent error AssertionError: Mempool sync timed out:
If it doesn't solve the issue completely, we may also need PR17770, PR18345 and PR18704. These 3 PRs were previous unsuccesful attempts to solve this issue by Core, maybe they are useless or maybe they are part of the solution.

This is a backport of Core PR18752

Test Plan: ninja && test/functional/test_runner.py mempool_reorg

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8436