Add test coverage for oversized block messages

Summary: Oversized blocks are treated differently from bitcoin core, so extra test coverage near boundaries is warranted.

Test Plan:

./test/functional/test_runner.py p2p_invalid_messages

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien, PiRK

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien, PiRK

Subscribers: PiRK, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16345