Dramatically speed up the abc_p2p_compactproofs test

Summary: This goes from 74s to 21s on my machine. This will help remove some of the flakiness or at least make it easier to debug.

Test Plan:

./test/functional/test_runner.py abc_p2p_compactproofs

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D14798