[tests] show the proofid when representing a proof

Summary: This would have made it easier to debug the issue fixed by D13099, to verify that the received msg_avaproof was indeed the one corresponding to the sent msg_getdata before comparing the timestamps.

Test Plan: Make a test involving a msg_avaproof (e.g. abc_p2p_compactproofs) fail and inspect the log for a msg_avaproof.

Reviewers: #bitcoin_abc, sdulfari, Fabien

Reviewed By: #bitcoin_abc, sdulfari, Fabien

Subscribers: Fabien, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D13100