test: improve msg sends and p2p disconnections in p2p_invalid_messages
test: improve msg sends and p2p disconnections in p2p_invalid_messages

- call disconnect_p2ps() outside of the assert_debug_log scopes
- send messages directly from the p2p conn rather than via nodes[0].p2p
- add an assertion


Former-commit-id: 061aede7d1fac94f2e4bfcc4c5b52b55cc5924f2
syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin