Add << operator overload for PeerMessagingState

Summary:
Not having this causes BOOST_CHECK_EQUAL() to fail on sanitizer builds when comparing
the enum.

Fixes T764, T763, and T762

Test Plan:

ninja check

Run sanitizer builds

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Subscribers: jasonbcox, deadalnix

Maniphest Tasks: T764, T763, T762

Differential Revision: https://reviews.bitcoinabc.org/D5470