tests: add tests for invalid P2P messages

E.g., ensure that we can't DoS a node by sending it a bunch of large,
unrecognized messages.

test/functional/test_runner.py

@@ -139,6 +139,7 @@ BASE_SCRIPTS = [
     'mining_prioritisetransaction.py',
     'p2p_invalid_locator.py',
     'p2p_invalid_block.py',
+    'p2p_invalid_messages.py',
     'p2p_invalid_tx.py',
     'feature_assumevalid.py',
     'example_test.py',