test: use inventory type constant MSG_CMPCT_BLOCK

Summary:
Bitcoin ABC already had such a constant, so this just renames it to use
the same as Core and uses it in p2p_compactblocks.py

This is a backport of Core PR18764 [2/3]
https://github.com/bitcoin/bitcoin/pull/18764/commits/b35e1d2471c3d9e0b6cc4b94f4726a7e6ea32206

Depends on D9141

Test Plan: ninja check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9142