Couldn't find a test for this and it seems something we should test, so I wrote one.

Commits:

test: Remove confusing cast to same type (int to int)

test: Check that invalid peer traffic is accounted for

This is a backport of core#20028

Backport notes:

• we ban for a bad checksum (see D2289), so the change in test_checksum is not applicable
• the change in indentation for self.nodes[0].disconnect_p2ps() on line 144 was supposed to be done in D9530. It does not affect the behavior, it just moves a cleanup disconnection outside the assertion scope

test/functional/test_runner.py p2p_invalid_messages