test: Fix intermittent error in mempool_reorg
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 [[https://github.com/bitcoin/bitcoin/pull/17770 | PR17770]], [[https://github.com/bitcoin/bitcoin/pull/18345 | PR18345]] and [[https://github.com/bitcoin/bitcoin/pull/18704 | 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 [[https://github.com/bitcoin/bitcoin/pull/18752 | 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

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