[test] Overhaul p2p_compactblocks.py

Summary:
backport of core#15660 and partial backport of core#20799
https://github.com/bitcoin/bitcoin/pull/20799/commits/cba909eaf938a775a9bd2dd994d06aba175e8713

Note that in our codebase test_node, ex_softfork_node and old_node are identical, as we use only version 1 compactblocks and NODE_NETWORK is the default services for TestP2PConn. As a result some tests were duplicated for no good reason. Removing this duplication divides the test time by 2.

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14872